Please explain in the README how to use the external copilot-language-server

copilot.vim bundles it, but some people don't like bundling and would like to use standalone copilot-language-server package.
[Repology](https://repology.org/projects/?search=copilot-language-server) shows that NixOS already has such package, and I am also going to create one for FreeBSD.

Is there an easy way to switch between the bundled copilot-language-server and external copilot-language-server ?

Command or setting that can switch it persistently?

從 README 開始，檢查 copilot.vim 目前如何選取其隨附的 copilot-language-server。確認現有的命令或設定是否支援外部可執行檔，然後記錄用於在隨附套件與獨立套件之間切換的持久設定；README 應涵蓋這兩種使用情境。

由索引模型根據 Issue 內容生成。