Add a lock around parse, to prevent concurrent access to the underlying parse tree.

[migueldeicaza]

The parse method in TreeSitterInternalLanguageMode is invoked directly by the
TextInputView.string.setter method, which can happen concurrently with the
calls to parse(_ text: NSString, completion: @escaping ((Bool) -> Void)) which happen to
queue things into a dedicated thread.

This causes a double-free of the underlying structures and assorted other crashes.

I caught this using the "address sanitizer" as I was trying to use my SwiftUI wrapper for
Runestone, which would trigger this event.

[simonbs]

Thanks a ton! I’ve been postponing fixing this for waaaay too long. It didn’t occur to me that that issue could be addressed this simply/elegantly 😄

[migueldeicaza]

Looks fine to me.