Closes #60761

Follow-up to #60970, which fixed this for `typescript-language-server`.
`vtsls` had the same problem, we only checked that the workspace tsdk
directory exists, but TypeScript 7 ships it without `tsserver.js`,
causing vtsls to error on startup.

Now we do the same `tsserver.js` check, so vtsls silently falls back to
its bundled TypeScript (see #60951).

Release Notes:

- Fixed vtsls showing a "doesn't point to a valid tsserver install"
error on startup in projects using TypeScript 7. For full TypeScript 7
support, install the `tsgo` extension to use TypeScript's native
language server.