Feat: Adds Ability to Merge MR (#147)

This adds the ability to merge an MR from within `gitlab.nvim` directly. If the reviewer is open, it'll be closed automatically. Users may configure whether they'd like to squash commits on the merge, as well as whether they'd like to delete the original source branch on a merge.

If squashing, users are prompted to provide an optional custom squash message for the squash commit.

lua/gitlab/reviewer/init.lua

@@ -22,6 +22,9 @@ M.init = function()
   M.open = reviewer.open
   -- Opens the reviewer window
 
+  M.close = reviewer.close
+  -- Closes the reviewer and cleans up
+
   M.jump = reviewer.jump
   -- Jumps to the location provided in the reviewer window
   -- Parameters: