Typechecker: Allow easier modification of standard library

[WardBrian]

See previous discussions in https://github.com/stan-dev/stanc3/pull/1182 and https://github.com/stan-dev/stanc3/pull/1184. In short, it is difficult to use the current compiler architecture with anything other than the signatures built in to Stan's math backend, including add things (like in Torsten) and replacing it entirely (like deepppl)

This is important to consider when thinking about https://github.com/stan-dev/stanc3/issues/1373 as well