v1.1.0: Replace Pydantic with custom validation logic, restore v0.1.5 config and registry algorithm

Pydantic v2 is stricter than v1 and I've been unable to replicate the behaviours we had in the config.

I had also attempted to refactor the library to make it easier to port, but this seems to have also introduced regressions.

To finally resolve this I'm dropping support for Pydantic in favour of a relatively small amount of custom validation logic. This should finally unblock spaCy and Thinc.