tools: add mailmap support for Co-authored-by tags

Support `.mailmap` for manually added `Author:` and
`Co-authored-by:` tags.

PR-URL: https://github.com/nodejs/node/pull/26383


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>