Part of https://github.com/zed-industries/zed/issues/22606

Before, `tsdk_path` for vtsls and typescript-language-server
unconditionally set a `tsdk`/`tsserver` property for the corresponding
language server, even if there were no such directory at all.
Instead, make the corresponding code to omit such property if it was not
found on the FS.

Release Notes:

- Fixed "The path /.../tsserver.js doesn't point to a valid tsserver
install. Falling back to bundled TypeScript version." pop-up appearing