refactor: rename Form to form_cls

[agoose77]

This renames Content.Form to Content.form_cls, which is better in line with the PEP8 naming conventions

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/agoose77-refactor-rename-form/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1976 (efcafea) into main (8273e2e) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/bitmaskedarray.py	69.33% <100.00%> (ø)
src/awkward/contents/bytemaskedarray.py	88.46% <100.00%> (ø)
src/awkward/contents/content.py	75.93% <100.00%> (ø)
src/awkward/contents/emptyarray.py	72.58% <100.00%> (ø)
src/awkward/contents/indexedarray.py	77.56% <100.00%> (ø)
src/awkward/contents/indexedoptionarray.py	88.67% <100.00%> (ø)
src/awkward/contents/listarray.py	90.50% <100.00%> (ø)
src/awkward/contents/listoffsetarray.py	79.79% <100.00%> (ø)
src/awkward/contents/numpyarray.py	90.44% <100.00%> (ø)
src/awkward/contents/recordarray.py	84.53% <100.00%> (ø)
... and 3 more

[jpivarski]

This is a good idea; let's merge it!