Cache DocumentDiagnosticAnalyzer results by version stamp to avoid redundant typecheck on unchanged documents

[xperiandri]

Fixes #20120

Summary

FSharpDocumentDiagnosticAnalyzer.GetDiagnostics previously recomputed syntax/semantic diagnostics (parse, typecheck, UnusedParentheses) on every crawler pass, even when the document text and project state had not changed since the last computation.

This adds a version-stamp-aware cache keyed by struct (DocumentId * DiagnosticsType), storing the last computed (textVersion, projectVersion, ImmutableArray<Diagnostic>) tuple:

• For Syntax diagnostics, only the document textVersion is tracked (projectVersion uses VersionStamp.Default).
• For Semantic diagnostics, both textVersion and document.Project.GetDependentVersionAsync() are tracked.
• If the current versions match the cached entry, the cached diagnostics are returned directly, skipping parse/typecheck/UnnecessaryParenthesesDiagnosticAnalyzer work entirely.

This mirrors the versioned-cache pattern used for referenced-project compilation emission in FSharpProjectOptionsManager.fs (emitCache).

Testing

• dotnet build vsintegration/src/FSharp.Editor/FSharp.Editor.fsproj -c Debug — succeeded.
• Deployed to RoslynDev hive (Build.cmd -c Debug -deployExtensions) and validated with a CPU trace of devenv.exe.