Rewrite XAML build to use a Roslyn source generator

[pchaurasia14]

Rewrite the XAML build process to use a Roslyn code generator instead of GenerateTemporaryTargetAssembly and related tasks.

[lindexi]

Looking forward to this improvement

[miloush]

What would be the benefits of spending time and resources on this?

[lindexi]

@miloush Such as remove the *_wpftmp.csproj files.

[miloush]

@lindexi why is that a problem? Is there something we cannot currently do that a source generator would fix?

[lindexi]

@miloush Some friends are troubled by the fact that sometimes the *_wpftmp.csproj file cannot be deleted automatically. And I think we can fix this without source generator...

[omariom]

What would be the benefits of spending time and resources on this?

Potentially faster builds.

[lindexi]

@miloush Maybe can fix #5711

[lindexi]

Maybe can fix #4544

I've tried to fix this before without success. One of the reasons is that it is difficult to change the current logical code.

[miloush]

I went through the code and this is a big task with a lot of test cases needed. If someone goes ahead, please make sure it works with VB.NET projects. (And obviously we lose other languages with CodeDom providers but I am not extremely concerned about those.)