* Move all PublicAPI.Unshipped.txt files to PublicAPI.Shipped.txt
- we have now shipped this API surface
* Add eng/PublicAPI.empty.txt file
- make it easier to add new implementation projects
* Add missing public API baseline files
- #26784 part 1/2
- a few unexpected files turned up missing beyond the #26784 list…
* Reference Microsoft.CodeAnalysis.PublicApiAnalyzers more often
- #27484 part 2/2
- emit errors when API baseline files are missing
- update categorization of some projects
- specification tests are not implementation projects
- but _were_ project reference providers (keep that)
- correct `$(IsTestAssetProject)` setting for Razor shims et cetera
- correct `$(IsSampleProject)` setting for casing of some Sample directories
- Microsoft.AspNetCore.Analyzer.Testing should be treated as test asset
- add `$(AddPublicApiAnalyzers)` property to support overrides
- e.g. for `msbuild` task projects and tools
nit:
- remove useless `$(IsTestProject)` setting in a specification test project
- wrap some long `Condition`s
* Do not run public API analyzer in `RazorCoreCompile` target
* Remove old `$(EnableApiCheck)` settings
- property does nothing
- also remove a dangling ApiCheck comment
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