[dotnet-watch] Host MSBuild in-process instead of shelling out

[pranavkm]

This has a few advantages including possibly faster builds in a rude edit, as well as a more robust API to interact with MSBuild

Things to do as part of this:

• Update scoped css support to query MSBuild project system about what the generated file looks like
• Updated static files support to query MSBuild for what the paths would look like rather than constructing it manually

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We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[pranavkm]

Notes from @ladipro's work: dotnet/msbuild#6254
The actual changes were done in: dotnet/sdk#16577

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.