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.

README.md

@@ -96,6 +96,9 @@ require("gitlab").setup({
       delete_comment = "dd",
       reply_to_comment = "r",
       toggle_node = "t",
+      position = "left", -- "top", "right", "bottom" or "left"
+      size = "20%", -- Size of split
+      relative = "editor" -- Position relative to "editor" or "window"
     },
     dialogue = { -- The confirmation dialogue for deleting comments
       focus_next = { "j", "<Down>", "<Tab>" },