For some reason, `axSub0R` was left out of `typeNatCoAxiomRules` in
`TcTypeNats`, which led to disaster when trying to look up `Sub0R` from
an interface file, as demonstrated in #14934.

The fix is simple—just add `axSub0R` to that list. To help prevent
an issue like this happening in the future, I added a
`Note [Adding built-in type families]` to `TcTypeNats`, which
contains a walkthrough of all the definitions in `TcTypeNats` you
need to update when adding a new built-in type family.

Test Plan: make test TEST=T14934

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #14934

Differential Revision: https://phabricator.haskell.org/D4508