Use the checker's document source for script snapshots

[nojaf]

Description

This fixes points 1 to 3 of #20750. FsAutoComplete wants to stop installing a process-wide FileSystem shim and give each checker its own view of the files (ionide/FsAutoComplete#1555). For scripts, DocumentSource.Custom did not work well enough for that yet.

• The checker's document source is used. FSharpChecker.GetProjectSnapshotFromScript fell back to DocumentSource.FileSystem when no documentSource was passed, also when the checker was created with DocumentSource.Custom. The TransparentCompiler's GetProjectOptionsFromScript always passed DocumentSource.FileSystem. Both now use the document source of the checker.
• Stable versions. FSharpFileSnapshot.CreateFromDocumentSource used DateTime.Now.Ticks as the version, so every call gave unchanged files a new version and the TransparentCompiler caches missed. This hit the #loaded files of a script and every FSharpProjectOptions based call on a TransparentCompiler created with DocumentSource.Custom. It now gets the text when the snapshot is created and uses its checksum as the version. Files read from disk keep the last write time.
• None reads from disk. When the custom source returned None, getting the source failed with "Couldn't get source for file". The file is now read from disk, as the BackgroundCompiler does (FSharpSource.fs).

API change: FSharpFileSnapshot.CreateFromDocumentSource now returns Async<FSharpFileSnapshot>, because the custom source is async and the version needs the text. The type is marked experimental and nothing outside FCS calls it. Surface area baseline is updated.

A custom source is now called for every file each time a snapshot is created, not only when a file needs parsing. Before, those calls missed the caches and reparsed everything anyway.

Not in this PR (see this comment):

• The script closure still reads #loaded files through the global FileSystem (ScriptClosure.fs, ClosureSourceOfFilename). This was already known in Ensure script load closure is always executed for GetProjectSnapshotFromScript #16880 (comment), where @0101 suggested a two-step approach: first gather the loaded files, then take their snapshots as input.
• The ScriptClosure cache is keyed on the root script only, so a cached closure is not updated when only a #loaded file changes (review comment).

documentSource on FSharpChecker.Create is still marked "likely to be removed in the future". If that is the plan, the question from #20750 still stands: how should an editor give the TransparentCompiler the text of open files that a script #loads?

Part of #20750

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

• Release notes entry updated:

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  Information about the release notes entries format can be found in the documentation.
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  • More inlines for Result module. (PR #16106)
  • Correctly handle assembly imports with public key token of 0 length. (Issue #16359, PR #16363)
    *while! (Language suggestion #1038, PR #14238)

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Tests: three new theories in tests/FSharp.Compiler.ComponentTests/FSharpChecker/TransparentCompiler.fs, each run with the BackgroundCompiler and the TransparentCompiler. All six fail on main and pass with this change.

[github-actions]

✅ Release notes checked

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.200.md