Feat: Create Merge Request

[harrisoncramer]

This MR will allow you to create a merge request for the current branch without leaving Neovim.

This was previously deferred as a feature since the glab CLI tool already supports this, however, being able to do this inside of the plugin is core to resolving/merging/managing MRs.

This required a slight refactor of the Go server, which previously made a call to get the MR info at startup every time. Since the MR will not necessarily be created now at startup, we have moved that call into a middleware that's called prior to every route that requires the data, assuming that the mergeId property has not already been initialized in the client state.

The refactor therefore moves all routes that require the additional merge request call under the /mr route prefix.

[harrisoncramer]

This MR will address #41