VS: initialize package in the background

[majocha]

Resolves #18623, fixes #18904

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
vsintegration/src	docs/release-notes/.VisualStudio/18.0.md

[majocha]

Works ok in experimental instance. It seems we don't need the UI thread during package init.
This must wait because currently the vsix cannot be installed

[T-Gro]

Thanks for taking this up @majocha .
This has the potential to speed up solution opening, so I am really glad the new API model allows us to make this split.

Lets merge this in after it can be locally tested.

[majocha]

I included fix for #18904. The setting was never set because the Initialize method was never called. We could extract the fix to another PR if needed.

[T-Gro]

I included fix for #18904. The setting was never set because the Initialize method was never called. We could extract the fix to another PR if needed.

Excellent for finding that out, @majocha . No need to extract it, we can keep it as a single PR bere.

[majocha]

I'm testing this PR atm. It seems fine now. FSI initializes ok. If there are any problems, (for example deadlocks at startup) it's a matter of moving some things from the background to foreground init.

[T-Gro]

I am going to merge this as it fixes an important user-facing regression with the solution crawler.
(especially on large solutions this would degrade the experience a lot)

@majocha : You are our hero 👍 !

[T-Gro]

/backport to release/dev18.0

[github-actions]

Started backporting to release/dev18.0: https://github.com/dotnet/fsharp/actions/runs/18745823459

[github-actions]

@T-Gro backporting to "release/dev18.0" failed, the patch most likely resulted in conflicts:

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: vs initialize in background
Using index info to reconstruct a base tree...
M	vsintegration/src/FSharp.Editor/LanguageService/LanguageService.fs
Falling back to patching base and 3-way merge...
Auto-merging vsintegration/src/FSharp.Editor/LanguageService/LanguageService.fs
CONFLICT (content): Merge conflict in vsintegration/src/FSharp.Editor/LanguageService/LanguageService.fs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 vs initialize in background
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!