feat(lsp): Explicit type annotation code action

[alex-snezhko]

[marcusroberts]

Great work Alex! A really useful feature to add too, thank you!

[ospencer]

Two questions for you: what's with the version additions, and what happens if you use this code action on something that already has an annotation?

[alex-snezhko]

@ospencer I got rid of the version stuff and it doesn't look like it made a difference actually (at least in VSCode's LSP client); I saw in the LSP documentation that the version was one of the response properties so that's why I implemented it initially.

The code action dialog should no longer show if it's tried on something that's already annotated; I realized that there was actually a flaw in my implementation which sometimes showed the code action dialog on something that was already annotated, but I patched it.

[ospencer]

Looks great!