Feat: Configure discussion tree split width and position

This MR improves the configurability of the plugin by using Nui's split
system rather than managing the splits ourselves. This lets the user
configure both the positioning of the discussion tree and it's width,
and reduces overall maintenance of split management for the plugin.

lua/gitlab/init.lua

@@ -134,9 +134,14 @@ M.setPluginConfiguration = function(args)
     error("The .gitlab.nvim project file's 'project_id' must be number")
   end
 
-  -- Configuration for the plugin, such as port of server
+  -- Configuration for the plugin, such as port of server, layout, etc
   state.PORT = args.port or 21036
   state.LOG_PATH = args.log_path or (vim.fn.stdpath("cache") .. "/gitlab.nvim.log")
+  state.DISCUSSION_SPLIT = {
+    relative = args.discussion_tree and args.discussion_tree.relative or "editor",
+    position = args.discussion_tree and args.discussion_tree.position or "left",
+    size = args.discussion_tree and args.discussion_tree.size or "20%",
+  }
 
   return true
 end