Make opening the settings file more robust (microsoft#1841)

* Make opening the settings file more robust

This fixes two issues.

 * Opens the assigned default application regardless of its configuration.
   Gvim for example only reacts to the "edit" verb so when selected as default application won't open.
   Using nullptr results in using the first specified application.
   This fixes microsoft#1789
 * If no application is assigned for json files fall back to notepad

 See https://docs.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-shellexecutea for more details
 especially why the result code checking is so horrific.

* Fix c-style cast

src/cascadia/TerminalApp/App.cpp

@@ -445,7 +445,12 @@ namespace winrt::TerminalApp::implementation
         co_await winrt::resume_background();
 
         const auto settingsPath = CascadiaSettings::GetSettingsPath();
-        ShellExecute(nullptr, L"open", settingsPath.c_str(), nullptr, nullptr, SW_SHOW);
+
+        HINSTANCE res = ShellExecute(nullptr, nullptr, settingsPath.c_str(), nullptr, nullptr, SW_SHOW);
+        if (static_cast<int>(reinterpret_cast<uintptr_t>(res)) <= 32)
+        {
+            ShellExecute(nullptr, nullptr, L"notepad", settingsPath.c_str(), nullptr, SW_SHOW);
+        }
     }
 
     // Method Description: