Fixed and completed implementation of multitargeted project dependency system

[Arlodotexe]

This PR:

• Closes Sample App Heads need to be able to load libraries based on Platform #182
• Closes Sample projects don't allow for WinUI 2/3 dependencies #165
• Uses the branch llama/markdown as a base (see [Sample App] Markdown Renderer Polyfill #151)
• Enables markdown on UWP and WinAppSdk as a side effect of the fixes
• Enables cleanly defining WinUI 2/3 (native or uno) dependencies under:
  • All deployable heads (new)
  • Sample projects (new)
  • Library projects (fixed a major bug)
• Fixes an issue with UseTargetFrameworks.ps1 where it could only be used when in the same directory
• Consolidates MSBuild bools to lowercase.

[michael-hawker]

@Arlodotexe looks like a build failure for the template. Can you explain what the Library projects (fixed a major bug) bug was here?

[michael-hawker]

I think my biggest question is I don't see where the Dependencies.props is getting referenced for inclusion anywhere?

[Arlodotexe]

Can you explain what the Library projects (fixed a major bug) bug was here?

@michael-hawker The switch between WinUI 2 and 3 in #139 used the property WinUIMajorVersion to decide between package references. However, in the MSBuild step that evaluates property values, TargetFramework is not defined (see here for my notes).

[michael-hawker]

We need a guard for uno around this:

  D:\a\Labs-Windows\Labs-Windows\template\lab\samples\ProjectTemplate.Wasm\obj\Debug\net5.0\g\XamlCodeGenerator\ToolkitDocumentationRenderer_aee9f0e9748a2e19cff6f84be8470785.g.cs(819,6): warning Uno0001: Windows.UI.Xaml.Controls.TextBlock.IsTextSelectionEnabled is not implemented in Uno [D:\a\Labs-Windows\Labs-Windows\template\lab\samples\ProjectTemplate.Wasm\ProjectTemplate.Wasm.csproj]

[Arlodotexe]

It's working locally, strange. I'll resolve the compiler errors soon.

So far in this PR, I've focused on implementing the feature using things I know 100% will work as expected, which resulted in needing to use 3 files instead of just 1 for PackageReferences.

However, using my notes here, I might have a way to combine them all into 1 file again while avoiding specific MSBuild quirks that have caused issues in the past. I'm testing it locally first to make sure it works.

[Arlodotexe]

Oddly, even though the syntax was exactly the same as our usage on TextBlock in ToolkitSampleRenderer:

-- and even though the property IsTextSelectionEnabled definitely exists on both TextBlock and MarkdownTextBlock, using the win: prefix here breaks UWP builds.

[Arlodotexe]

Still working to resolve the build errors, locally before pushing.
Though I'm somehow getting a different error from what the CI is getting...

[Arlodotexe]

We'll be using #199 instead