feat: add overrideDefs function

[jedwards1211]

Solving benjamn/recast#1283 would be difficult because
it imports ast-types everywhere, and I would have to replace those references
with passing an ast-types fork as new function parameters all over the place.

This is the pragmatic solution, it would allow me to just replace all of the
ast-types exports with my own fork built from @babel/types (see
#823). So anywhere that imports
them would get my definitions that are up-to-date with babel.

Building defs from @babel/types or other parsers' metadata is the only
sustainable option for the community to have a reliable codemod system built on recast.

[jedwards1211]

Ugh this won't work the best either...I would have to make sure I override the defs before I import recast