ci: lint PR titles according to conventional commits

[agoose77]

This PR introduces a new workflow that runs on PRs:
https://github.com/amannn/action-semantic-pull-request

By default, this uses the commitizen (angular derivative) convention:

Note that some/all of the commits in this PR use a sentence-cased variant, because that was what I was used to. The new standard is lowercase prefixes.

[agoose77]

Good, the checks fail for maint:. Now try ci:...

[codecov]

Codecov Report

Merging #1615 (3907522) into main (9e17f29) will decrease coverage by 0.02%.
The diff coverage is 62.09%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/cuda/__init__.py	0.00% <0.00%> (ø)
src/awkward/_v2/_connect/numexpr.py	88.40% <0.00%> (ø)
src/awkward/_v2/_connect/pyarrow.py	88.46% <0.00%> (ø)
src/awkward/_v2/numba.py	93.47% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_avro_file.py	66.66% <0.00%> (ø)
src/awkward/_v2/operations/ak_local_index.py	100.00% <ø> (ø)
src/awkward/_v2/operations/ak_type.py	53.84% <0.00%> (ø)
src/awkward/_v2/operations/ak_transform.py	8.62% <8.62%> (ø)
src/awkward/_v2/contents/unionarray.py	86.16% <50.00%> (-0.11%)	⬇️
src/awkward/_v2/contents/bitmaskedarray.py	71.53% <53.84%> (+5.11%)	⬆️
... and 26 more

[agoose77]

@henryiii @jpivarski you both expressed an interest in this (by engaging in the discussion on Zoom)! Let me know if you are happy to merge this as-is, or want any changes.

[jpivarski]

So this one is

Lint PR / Validate PR title (pull_request) Successful in 3s

(I was wondering what form the title test would take, since it's not a git-level thing.)

Works for me. I can make this a requirement. I'll merge this now and make it a requirement on Wednesday, since everyone with an open PR will have to merge this branch.

[jpivarski]

Although I'll give @henryiii a chance to weigh in before actually merging.

[henryiii]

Seems fine to me, we could always remove it if it doesn't work out well for us.