VS: initialize package in the background

[majocha]

Description

Fixes #18623

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

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[github-actions]

❗ Release notes required

@majocha,

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Change path	Release notes path	Description
vsintegration/src	docs/release-notes/.VisualStudio/18.0.md	No release notes found or release notes format is not correct

[majocha]

Let's see if this is still needed.

[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.

[T-Gro]

For other readers -> this is the significant line.