Currently, extensions cannot have tests that call methods like
`label_for_symbol` and `label_for_completion`, because those methods
take a `LanguageServerId`, and that type is opaque, and cannot be
constructed outside of the `zed_extension_api` crate.

This PR makes it possible to construct those types from strings, so that
it's more straightforward to write unit tests for these LSP adapter
methods.

Release Notes:

- N/A