Add hook to check toml files

[alecandido]

The hook is one of the default ones, and essentially for free. It will mainly help keeping pyproject.toml formatted.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[alecandido]

Ok, I was joking, it's just verifying the syntax, it's not formatting anything.

To get the formatting part, we'd need something like:
https://github.com/toml-lang/toml/blob/dc5af251cd348ee053d9355359743767e3d0b123/.pre-commit-config.yaml#L2-L5
or
https://github.com/ComPWA/taplo-pre-commit
but they are not "for free", so I'm not going to propose them (for the time being).

[stavros11]

Maybe we could also propagate to other repos (if not already there).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (3a7a951) to head (a922b8b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1480   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files          81       81           
  Lines       11800    11800           
=======================================
  Hits        11792    11792           
  Misses          8        8           

Flag	Coverage Δ
unittests	99.93% <ø> (ø)

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[alecandido]

Maybe we could also propagate to other repos (if not already there).

That's exactly why I appointed you as a reviewer :)