Meta.yaml schema (implemented with traitlets!)

[cisaacstern]

A much lighter version of #134 as a starting place.

As I got into this, I realized we didn't need to sacrifice much of #134 after all, because almost everything there can be done with pure traitlets, saving us the Pydantic dependency!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (005951c) 96.11% compared to head (e6cefce) 96.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   96.11%   96.09%   -0.03%     
==========================================
  Files          14       15       +1     
  Lines         463      486      +23     
==========================================
+ Hits          445      467      +22     
- Misses         18       19       +1     

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[cisaacstern]

@jbusecke are in the final 2 hrs of our hack week here in LA, and I need this PR to finish the work I'm targeting to complete for him! The only failing check here is -0.03% on coverage:

Which I think is close enough! Everthing else passes and AFAICT (🤞 🤞 🤞 ) the public API is totally unchanged.

@ranchodeluxe and @yuvipanda please forgive me if this is too rash! I beg forgiveness to the PR review gods and generally don't endorse unreviewed merges! But the hack week goals are calling...

To be clear, I totally 100% encourage follow-on Issues/PRs if there are things here anyone doesn't like, and with that, I'll merge!