LSP: Support watch mode for tests

[stefnotch]

To implement denoland/vscode_deno#1132 , we need to hook up the file watching in the Deno LSP.

I had previously attempted to implement this here #27762 , but that PR is stalled. It is blocked on #29132 . The current file-watching-and-rerunning abstraction in Deno is too low level to easily be re-used.

Once that issue gets resolved, I would be more than happy to contribute a PR that actually works as intended.