[ci] Automatically label PRs from core team (#32100)

Adds a new `MAINTAINERS` file which contains github usernames of core
team members. This file serves as documentation for core team membership
and is also used to automatically label PRs from core.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/32100).
* #32101
* __->__ #32100