doc: document considerations for inclusion in core

Document the things that are considered when making the determination as
to whether something should or shouldn't be in core. This does not (yet,
at least) attempt to address *how* to include modules in core. (Should
it be in the Node.js code base or vendored in from a separate
repository?) It is limited to *whether* something should be in core or
not.

Closes: https://github.com/nodejs/TSC/issues/1041

PR-URL: https://github.com/nodejs/node/pull/40338
Fixes: https://github.com/nodejs/TSC/issues/1041


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>