Implement support for pull diagnostic requests on additional files

[tmat]

Depends on 1342106, which would enable us to dynamically register arbitrary files with the LSP client.

Once we have this capability we would register all additional files and respond to document pull diagnostics for these files. We would run document analysis (specifically, built-in analyzers such as EnC analyzer) on all source generated files on the request. We would then report all diagnostics that are returned from this analysis. The locations of these diagnostics would be expected to map back to the corresponding additional file via #line directives present in the source generated file.

We may find that the performance of this implementation is not good enough in which case we can optimize by establishing relationship between specific additional files and the corresponding source-generated files and only limiting analysis to the affected files.

This feature would enable diagnostics produced for source-generated files by any analyzer to be reported in additional files for any source generator that produces source code based on the content of additional files.

Specifically, it would address issues with reporting EnC rude edits for XAML files in MAUI apps (see 1577259). It would also allow us to remove custom pull diagnostic forwarding from Razor in future (once it unifies design-time and compile-time files).

[tmat]

17.4 for now but will need to be pushed out to 17.5 due to dependency on VS Platform APIs.

[tmat]

The LSP work item is scheduled for 17.5 Preview 2.