fix: harden typing in the code base; add pre-commit check; improve docs

[douglasdavis]

• fix various existing typing errors
• require mypy check with pre-commit
• add developer docs explaining that while typing is not a requirement in dask-awkward, if it's used in a signature we want to make sure it's checked

we've always had type hints throughout dask-awkward but it's never been formalized, this PR aims to clarify typing in the project.

[codecov-commenter]

Codecov Report

Merging #383 (3d68678) into main (8c9ff67) will decrease coverage by 0.72%.
The diff coverage is 94.44%.

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@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
- Coverage   93.21%   92.50%   -0.72%     
==========================================
  Files          24       24              
  Lines        3038     3043       +5     
==========================================
- Hits         2832     2815      -17     
- Misses        206      228      +22     

Files	Coverage Δ
src/dask_awkward/layers/layers.py	92.43% <100.00%> (ø)
src/dask_awkward/lib/core.py	92.71% <100.00%> (-0.01%)	⬇️
src/dask_awkward/lib/io/columnar.py	100.00% <100.00%> (ø)
src/dask_awkward/lib/io/json.py	88.70% <100.00%> (ø)
src/dask_awkward/lib/io/parquet.py	94.30% <100.00%> (ø)
src/dask_awkward/lib/optimize.py	96.41% <ø> (ø)
src/dask_awkward/lib/utils.py	87.50% <100.00%> (+0.14%)	⬆️
src/dask_awkward/lib/inspect.py	58.66% <71.42%> (+0.92%)	⬆️

... and 1 file with indirect coverage changes

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