Insert doc comments when typing via LSP

[davidwengier]

Fixes AB#1155667

Adds support to LSP for inserting doc comments during typing, both when typing '///' above a method/class, or when pressing enter within a doc comment.

You'll pretty much have to review commit-by-commit, since the first commit is a straight cut and paste of code into a service without modification. Clean up comes later to make reviewing it easier.

[dibarbet]

mostly looks good - if there are any areas in the doc comment service where you made material logic changes instead of mostly copy paste let me know

[allisonchou]

LGTM 🎉 (though I'd probably get another approval in addition just to be sure) All of my comments are nits, although I didn't look too closely at the move to service since I'm assuming it's just code being moved over.

[davidwengier]

if there are any areas in the doc comment service where you made material logic changes instead of mostly copy paste let me know

The only change of note with the existing code is that in the command handler, two of the operations did an Insert and two did a Replace. Now they all do a Replace so there was some tweaking of the spans to get that to work, but the existing command handler tests all pass without modification so I'm pretty confident :)

[davidwengier]

All of the PR feedback has been addressed