Initial WinUI2 GetStart project

[mikehoffms]

This repo hasn't had a project for GetStart WinUI 2 (UWP). This PR adds such a project.

This PR adds the resulting project that results from updating the steps in article "Get started with WebView2 in WinUI 2 (UWP) apps" https://learn.microsoft.com/en-us/microsoft-edge/webview2/get-started/winui2 - that article update is PR https://github.com/MicrosoftDocs/edge-developer/pull/2456 in "edge-developer" Docs repo.

.sln filename:

• MyUWPGetStartApp.sln
  • "GetStart" is better than "Sample", to differentiate.
  • Specifies the platform (UWP).
  • Future PRs should omit "My" and "App"; a future PR could name it "UWPGettingStarted.sln", "WinUI2GettingStarted.sln", or "WinUI2UWPGettingStarted.sln".

Existing GetStart .sln filenames:

• WPFSample.sln
• WebView2GettingStarted.sln
• WinForms_GettingStarted.sln
• WinUI_Sample.sln

Folder name:

• WinUI2_GettingStarted

Existing folder names, listed at https://learn.microsoft.com/en-us/microsoft-edge/webview2/get-started/get-started -

• WinUI3_GettingStarted
• WPF_GettingStarted
• WinForms_GettingStarted
• Win32_GettingStarted