Add support for compact tab sizing (#5787)

## Summary of the Pull Request

Really couldn't be more starightforward. MUX 2.4 added support for "compact" sized tabs. This PR (targeting the 2.4 PR currently, will move to `master` when that merges) enables users to specify `"tabWidthMode": "compact"` in their global settings to get this behavior.

## References
* #5778 - PR to move to MUX 2.4
* [microsoft-ui-xaml#2016](microsoft/microsoft-ui-xaml#2016) - the MUX PR for compact tab sizing.
* #597 - Tab sizing options?

## PR Checklist
* [x] I don't think we have an issue for this, though I could be wrong.
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

In this screenshot, I'm hovering over tab 2, but the ubuntu tab is focused:
![image](https://user-images.githubusercontent.com/18356694/81302365-e6ef4000-903f-11ea-9ce3-5f5ce92e5ba4.png)

In this screenshot, tab 2 is focused:
![image](https://user-images.githubusercontent.com/18356694/81302383-ea82c700-903f-11ea-9820-92348d5adc64.png)

doc/cascadia/SettingsSchema.md

@@ -19,7 +19,7 @@ Properties listed below affect the entire window, regardless of the profile sett
 | `showTerminalTitleInTitlebar` | _Required_ | Boolean | `true` | When set to `true`, titlebar displays the title of the selected tab. When set to `false`, titlebar displays "Windows Terminal". |
 | `showTabsInTitlebar` | Optional | Boolean | `true` | When set to `true`, the tabs are moved into the titlebar and the titlebar disappears. When set to `false`, the titlebar sits above the tabs. |
 | `snapToGridOnResize` | Optional | Boolean | `false` | When set to `true`, the window will snap to the nearest character boundary on resize. When `false`, the window will resize "smoothly" |
-| `tabWidthMode` | Optional | String | `equal` | Sets the width of the tabs. Possible values: `"equal"`, `"titleLength"` |
+| `tabWidthMode` | Optional | String | `equal` | Sets the width of the tabs. Possible values: <br><ul><li>`"equal"`: sizes each tab to the same width</li><li>`"titleLength"`:  sizes each tab to the length of its title</li><li>`"compact"`: sizes each tab to the length of its title when focused, and shrinks to the size of only the icon when the tab is unfocused.</li></ul> |
 | `wordDelimiters` | Optional | String | <code>&nbsp;&#x2f;&#x5c;&#x28;&#x29;&#x22;&#x27;&#x2d;&#x3a;&#x2c;&#x2e;&#x3b;&#x3c;&#x3e;&#x7e;&#x21;&#x40;&#x23;&#x24;&#x25;&#x5e;&#x26;&#x2a;&#x7c;&#x2b;&#x3d;&#x5b;&#x5d;&#x7b;&#x7d;&#x7e;&#x3f;│</code><br>_(`│` is `U+2502 BOX DRAWINGS LIGHT VERTICAL`)_ | Determines the delimiters used in a double click selection. |
 | `confirmCloseAllTabs` | Optional | Boolean | `true` | When set to `true` closing a window with multiple tabs open WILL require confirmation.  When set to `false` closing a window with multiple tabs open WILL NOT require confirmation. |
 | `startOnUserLogin` | Optional | Boolean | `false` | When set to `true` enables the launch of Windows Terminal at startup. Setting to `false` will disable the startup task entry. Note: if the Windows Terminal startup task entry is disabled either by org policy or by user action this setting will have no effect. |

doc/cascadia/profiles.schema.json

@@ -376,8 +376,9 @@
         },
         "tabWidthMode": {
           "default": "equal",
-          "description": "Sets the width of the tabs. Possible values include:\n -\"equal\" sizes each tab to the same width\n -\"titleLength\" sizes each tab to the length of its title",
+          "description": "Sets the width of the tabs. Possible values include:\n -\"equal\" sizes each tab to the same width\n -\"titleLength\" sizes each tab to the length of its title\n -\"compact\" sizes each tab to the length of its title when focused, and shrinks to the size of only the icon when the tab is unfocused.",
           "enum": [
+            "compact",
             "equal",
             "titleLength"
           ],

src/cascadia/TerminalApp/GlobalAppSettings.cpp

@@ -27,27 +27,34 @@ static constexpr std::string_view InitialPositionKey{ "initialPosition" };
 static constexpr std::string_view ShowTitleInTitlebarKey{ "showTerminalTitleInTitlebar" };
 static constexpr std::string_view ThemeKey{ "theme" };
 static constexpr std::string_view TabWidthModeKey{ "tabWidthMode" };
-static constexpr std::wstring_view EqualTabWidthModeValue{ L"equal" };
-static constexpr std::wstring_view TitleLengthTabWidthModeValue{ L"titleLength" };
 static constexpr std::string_view ShowTabsInTitlebarKey{ "showTabsInTitlebar" };
 static constexpr std::string_view WordDelimitersKey{ "wordDelimiters" };
 static constexpr std::string_view CopyOnSelectKey{ "copyOnSelect" };
 static constexpr std::string_view CopyFormattingKey{ "copyFormatting" };
 static constexpr std::string_view LaunchModeKey{ "launchMode" };
 static constexpr std::string_view ConfirmCloseAllKey{ "confirmCloseAllTabs" };
 static constexpr std::string_view SnapToGridOnResizeKey{ "snapToGridOnResize" };
+
+static constexpr std::string_view DebugFeaturesKey{ "debugFeatures" };
+
+static constexpr std::string_view ForceFullRepaintRenderingKey{ "experimental.rendering.forceFullRepaint" };
+static constexpr std::string_view SoftwareRenderingKey{ "experimental.rendering.software" };
+static constexpr std::string_view EnableStartupTaskKey{ "startOnUserLogin" };
+
+// Launch mode values
 static constexpr std::wstring_view DefaultLaunchModeValue{ L"default" };
 static constexpr std::wstring_view MaximizedLaunchModeValue{ L"maximized" };
 static constexpr std::wstring_view FullscreenLaunchModeValue{ L"fullscreen" };
+
+// Tab Width Mode values
+static constexpr std::wstring_view EqualTabWidthModeValue{ L"equal" };
+static constexpr std::wstring_view TitleLengthTabWidthModeValue{ L"titleLength" };
+static constexpr std::wstring_view TitleLengthCompactModeValue{ L"compact" };
+
+// Theme values
 static constexpr std::wstring_view LightThemeValue{ L"light" };
 static constexpr std::wstring_view DarkThemeValue{ L"dark" };
 static constexpr std::wstring_view SystemThemeValue{ L"system" };
-static constexpr std::string_view EnableStartupTaskKey{ "startOnUserLogin" };
-
-static constexpr std::string_view ForceFullRepaintRenderingKey{ "experimental.rendering.forceFullRepaint" };
-static constexpr std::string_view SoftwareRenderingKey{ "experimental.rendering.software" };
-
-static constexpr std::string_view DebugFeaturesKey{ "debugFeatures" };
 
 #ifdef _DEBUG
 static constexpr bool debugFeaturesDefault{ true };
@@ -322,6 +329,10 @@ TabViewWidthMode GlobalAppSettings::_ParseTabWidthMode(const std::wstring& tabWi
     {
         return TabViewWidthMode::SizeToContent;
     }
+    else if (tabWidthModeString == TitleLengthCompactModeValue)
+    {
+        return TabViewWidthMode::Compact;
+    }
     // default behavior for invalid data or EqualTabWidthValue
     return TabViewWidthMode::Equal;
 }