Support schema validation in `dotnet msbuild`.

[teo-tsirpanis]

The -validate CLI option is gated behind the FEATURE_XML_SCHEMA_VALIDATION constant, which is enabled only in .NET Framework, but the underlying APIs are available in modern .NET too.

This issue tracks adding this feature to the modern .NET flavor of MSBuild. It should be trivial (just removing the ifdefs, which I have done locally in a branch), unless I'm missing something.

[rainersigwald]

Have you found this useful? I have never heard of any use case for this feature, and am inclined to remove it from the netfx output rather than port it to core . . .

[teo-tsirpanis]

I never had to use it; I just noticed the disparity between the frameworks.

I'm not against removing it; XML schema validation can be performed with other tools. From what I saw, it validates only the project file and not any included target files, making implementing this on MSBuild less necessary.