Convert to AsyncPackage with background load

[madskristensen]

For performance reasons all extensions targeting VS2015+ should convert the Package class to AsyncPackage. See more here https://docs.microsoft.com/en-us/visualstudio/extensibility/how-to-use-asyncpackage-to-load-vspackages-in-the-background

It's as easy as doing exactly what your current Package.Initialize method does, but in the Package.InitializeAsync method, make the first line await JoinableTaskFactory.SwitchToMainThreadAsync();

[RussKie]

@sharwell is this something you have experience with?

[sharwell]

Yes, but I'm waiting for #4747 and #4799 to be completed before starting new work on the VS side of things.

[sharwell]

Also a decision needs to be made on #4752 (comment), since that could make this issue obsolete.