ci: check that pull request titles follow Conventional Commits

[The127]

A squash merge takes the title of the pull request as the commit message, and no hook sees that commit. The new pr title workflow fails a title that is not type: description without a scope, with the same pattern as the commit-msg hook.

release-please now titles its pull requests chore: release <version>, so they pass the check.

Docs: I checked them. CONTRIBUTING.md says the title of a pull request follows the same form.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.10%. Comparing base (56ff891) to head (b5cbb71).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   56.08%   56.10%   +0.02%     
==========================================
  Files         288      288              
  Lines        4761     4761              
==========================================
+ Hits         2670     2671       +1     
+ Misses       2091     2090       -1     

Flag	Coverage Δ
unit	56.10% <ø> (+0.02%)	⬆️

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