Fix wrong imports in oauth-client-node README

[KevSlashNull]

In the @atproto/oauth-client-node README code examples, this:

1. adds missing imports,
2. fixes the NodeOAuthClient import (because NodeOAuthClientOptions is just a type, not a constructor),
3. and adds async to some functions in the options, so they actually return Promises.

[matthieusieben]

Thanks

[dholms]

Not exactly sure what the linting error here is, but do you mind running a quick pnpm format @KevSlashNull?

[surfdude29]

Sorry for jumping in here, I think the linting error might have been in a different README and was fixed in 3c7bf81 ?

[KevSlashNull]

@dholms, I think @surfdude29 is right, so I rebased.