Problem: CompleteDone fires with reason="discard" even when the candidate
text was inserted and left in the buffer, because reason was determined
solely by the terminating keycode (Ctrl-Y).
Solution: Check compl_used_match to detect whether inserted
text remains in the buffer, and set reason="accept" accordingly.
Problem: there is no way to distinguish between user's explicit
completion stop/cancel and other automated reasons.
Solution: update "cancel" reason to be set only on explicit CTRL-e, and
set intentionally vague "discard" otherwise.
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.
