[Feat | Docs] Add typings for tree-sync

[timlindvall]

New feature proposal | Documentation

What?
Let's add a TypeScript type definition file for the tree-sync module! It'll be included with the package's files and referenced using the types property in package.json.

Why?

• TypeScript users don't have to define the module in their own repos, and they can get other IDE niceties out of the box (if their IDE knows how to reason about TypeScript).
• Provides public documentation of tree-sync's API.

(FYI: I'm already working on this at present - posting an issue ahead of a PR for visibility and feedback. Barring any objections I'll have an PR posted for review soon... ish. Thanks for considering!)