Fix MR Selection, Go Code Refactor (#358)

refactor: Refactors the Go codebase into a more modular and idiomatic approach
fix: require selection of specific MR when there are multiple targets for a given source branch
feat: Allows for the passing of Gitlab's filter options when choosing an MR, improves MR selection
feat: API to choose an MR from a list based on the provided username's involvement as an assignee/reviewer/author

This is a #MINOR release

lua/gitlab/actions/merge_requests.lua

@@ -45,6 +45,7 @@ M.choose_merge_request = function(opts)
       end
 
       vim.schedule(function()
+        state.chosen_target_branch = choice.target_branch
         require("gitlab.server").restart(function()
           if opts.open_reviewer then
             require("gitlab").review()