feat(complete): specify reason for CompleteDone

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.

src/nvim/vvars.lua

@@ -215,6 +215,7 @@ M.vars = {
         changed_window   Is |v:true| if the event fired while
                          changing window  (or tab) on |DirChanged|.
         status           Job status or exit code, -1 means "unknown". |TermClose|
+        reason           Reason for completion being done. |CompleteDone|
     ]=],
   },
   exception = {