Show commment on last line of range and update internals

[alexr00]

Part of #146510

[alexr00]

@rebornix feel free to review or not, as you wish :) I intend to merge this tomorrow (unless you have feedback to the contrary). I mostly wanted to give you a heads up on this change, which basically does two things:

1. Shows the comment on the last line of the range. This is better than showing on the first line of the range because it doesn't split range with the comment editor zone. This is also what GitHub does.
2. Wires up the internals of comments to support ranges instead of single lines, and exposes this through the "Add Comment on Current Selection" (formerly "Add Comment on Current Line") command. There's no new UX here, the command just now works on a selection instead of a line in order to take advantage of the multiline support.

[rebornix]

LGTM, only one suggestion for how we can avoid calculating the accurate comment range after editor model is modified.

[rebornix]

We used the zone widget position since previously we only track single line and it serves good enough as markers. To accurately track the movement of the comment range, we may want to add a decoration for commentThread.range in the editor and reveive the latest range of the decoration on action execution.

[alexr00]

This makes sense to me. I will leave as is for now though, since the decoration for a range requires some UX decisions that we have yet to make.