Use source generators to compile Razor files

[pranavkm]

This change replaces the use of RazorGenerate and RazorTagHelper tasks, and rzc, with a source generator that discovers tag helpers, code generates .razor and .cshtml files, and compiles the .razor files. .cshtml files are written to disk that are used during .Views.dll compilation.

Some notes:

• The SDK continues to ship rzc, and tasks to support < net6.0 apps. It might be possible to use the source generator in 3.1 and later apps, but there are some implications to doing this which need further thought and discussion. I've punted on this for the time being.
• For 6.0 apps, a lot of the Razor pipeline that was previously driven by MSBuild is now part of a single source generator.
• Discovering tag helpers afresh on every build is really expensive. It's much more effective to be able to cache some results and use it in consecutive builds. One strategy that works very well for this is to partition the results so that tag helpers from referenced assemblies are cached but not the ones from the app itself. Right now we're caching to disk which is not recommended., We'll work on getting APIs that allow caching in memory to avoid the additional serialization costs.

Open issues:

• Figure out how source generators work in the IDE. Right now every key stroke results in the source generator running completely in the IDE. Caching results helps, but we need to do better. This is a problem for all source generators that the compiler team is aware of.
• Make razor participate in EnC. There's some WIP work in this PR that hasn't been shown to work, but I'm leaving it in there to make my life easier.
• Assess SDK change impact. The RazorSDK no longer invokes a bunch of targets that were previously part of the build pipeline . This was observed as part of the IdentityUI project that was part of our repo, but it's not clear how this affects projects in the wild.

	Before	After	%
Cold start	3735 ms	2740 ms	26
Warm build	1321 ms	1074 ms	19
No-op build	323 ms	316 ms	2
Change to .cs file	713 ms	600 ms	16
Change to .cshtml file	632 ms	575 ms	9
Change to .razor file	641 ms	576 ms	10
Change to .razor file to add parameter	774 ms	591 ms	23

[petr-fleischmann]

"!isComponent" is redundant check now. if path.EndsWith(".cshtml", StringComparison.OrdinalIgnoreCase) == true then path.EndsWith(".razor", StringComparison.OrdinalIgnoreCase) = false.