lsp: Watch paths outside of worktrees at language servers request (#17499)

Another stab at https://github.com/zed-industries/zed/pull/17173, this
time fixing the segfault found in
https://github.com/zed-industries/zed/pull/17206

Release Notes:

- Improved language server reliability in multi-worktree projects and
monorepo. We now notify the language server more reliably about which
files have changed.