chore: rough demo of promise usage

[wemeetagain]

• do not merge, this is just for demo purposes
• This shows how work can be delegated to a background worker and have a promise returned which is resolved when the work is complete
  • the include/ folder is just the napi headers
  • this just uses https://nodejs.org/api/n-api.html#simple-asynchronous-operations
  • look at the additions to zig/leveldb.zig, its quite verbose, but it goes like:
    • leveldb_db_get_promise gets the input, creates+returns the promise and creates+queues the async work
    • leveldb_db_get_execute_callback is where the "heavy work / io" is performed (can't use napi in this fn)
    • leveldb_db_get_complete_callback is where the result is wrapped / napi-ified
• the added test shows that (at least for this microbenchmark) that the sync version is faster than even the time to return the promise in the async version
  • this shows that we need to carefully apply this pattern only where it "makes sense", eg where the work takes more than ~0.1ms