Add a WT_SETTINGS_DIR env variable that portable profiles can use (#1…

…6949)

Basically, title. 

It'd be a neat idea for portable installs of the Terminal to reference
files that are right there in the portable install.

This PR adds a `WT_SETTINGS_DIR` var to Terminal's own env block. This
allows us to resolve profiles relative to our own settings folder.

Closes #16295

src/cascadia/TerminalApp/AppLogic.cpp

@@ -149,6 +149,11 @@ namespace winrt::TerminalApp::implementation
         _languageProfileNotifier = winrt::make_self<LanguageProfileNotifier>([this]() {
             _reloadSettings->Run();
         });
+
+        // Do this here, rather than at the top of main. This will prevent us from
+        // including this variable in the vars we serialize in the
+        // Remoting::CommandlineArgs up in HandleCommandlineArgs.
+        _setupFolderPathEnvVar();
     }
 
     // Method Description:
@@ -720,4 +725,14 @@ namespace winrt::TerminalApp::implementation
         return TerminalApp::ParseCommandlineResult{ winrt::to_hstring(_appArgs.GetExitMessage()), r };
     }
 
+    // Function Description
+    // * Adds a `WT_SETTINGS_DIR` env var to our own environment block, that
+    //   points at our settings directory. This allows portable installs to
+    //   refer to files in the portable install using %WT_SETTINGS_DIR%
+    void AppLogic::_setupFolderPathEnvVar()
+    {
+        std::wstring path{ CascadiaSettings::SettingsPath() };
+        auto folderPath = path.substr(0, path.find_last_of(L"\\"));
+        SetEnvironmentVariableW(L"WT_SETTINGS_DIR", folderPath.c_str());
+    }
 }

src/cascadia/TerminalApp/AppLogic.h

@@ -112,6 +112,8 @@ namespace winrt::TerminalApp::implementation
         void _RegisterSettingsChange();
         fire_and_forget _DispatchReloadSettings();
 
+        void _setupFolderPathEnvVar();
+
 #ifdef UNIT_TESTING
         friend class TerminalAppLocalTests::CommandlineTest;
 #endif