This PR updates Copilot to use the official
[`@github/copilot-language-server`](https://github.com/github/copilot-language-server-release).

It's [available on
npm](https://www.npmjs.com/package/@github/copilot-language-server), so
we're installing it from there.

I tested it out locally and it seemed to be working as expected.

Release Notes:

- Updated Copilot to use the official
[`@github/copilot-language-server`](https://github.com/github/copilot-language-server-release).