[generate-changelog] Add `--format` option (#34992)

Adds a new `--format` option which can be `text` (default), `csv`, or
`json`.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34992).
* #34993
* __->__ #34992