Consider making `DecodeError` and `ValidationError` inherit from `ValueError`

[vytas7]

Description

In many scenarios and libraries one assumes that validation errors inherit from ValueError, however, msgspec's exception classes do not. Would it be possible to make DecodeError and ValidationError subclasses of ValueError somewhere in the hierarchy?

[jcrist]

Makes sense to me, happy to make this change. This would be a breaking change of course, but since we're still in 0.* that shouldn't be unexpected.