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.
This MR represents a major refactor of the Go codebase, as well as introducing tests for the handlers. The MR also introduces an endpoint to shutdown or restart the Go server, which may be useful for clients who want to refresh the state of the plugin after checking out branches. Finally, this MR adds a contributing document for users who want to make feature changes.
