feat: add `validators.py` to validate that templates can be successfully built

[ycexiao]

Problem

We need to validate that the templates are correct in an automatic way.

Proposed solution

Add validators.py,

[ycexiao]

@sbillinge , should we put validator.py in the hook?

[sbillinge]

This is relevant when people add templates in the future, so we may want to run them in the merge workflow for a pr on this repo.