Problem: `CompleteDone` currently does not specify the reason for why completion was done, which is problematic for completion plugins as they cannot know whether the event was triggered due to the completion being canceled, accepted, or for some other reason.
Solution: Add a `reason` key to `v:event`, which is set by `CompleteDone` to indicate why completion ended.
This commit implements a new TermRequest autocommand event and has Neovim
emit this event when children of terminal buffers emit an OSC or DCS sequence
libvterm does not handle.
The TermRequest autocommand event has additional data in the
v:termrequest variable.
Co-authored-by: Gregory Anders <greg@gpanders.com>