investigate if we need async NAPI APIs

[Omar Tawfik]

The Language.parse() API (and possibly others) should be turned async returning a Promise<T>. This is important for clients like an LSP that shouldn't block the main event loop over parsing that can happen in the background.

More info:

• https://napi.rs/docs/concepts/async-fn
• https://napi.rs/docs/concepts/async-task
• https://napi.rs/docs/concepts/threadsafe-function