Add type hints to skll.experiments module

[desilinguist]

• Add new custom types to skll.types.
• Fix buggy type hint in skll.config.utils.
• Add type hints to all files under skll.experiments.

[codecov]

Codecov Report

Patch coverage: 98.52% and project coverage change: +0.01 🎉

Comparison is base (a3f45d5) 95.24% compared to head (c17da1e) 95.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #732      +/-   ##
==========================================
+ Coverage   95.24%   95.26%   +0.01%     
==========================================
  Files          29       29              
  Lines        3449     3461      +12     
==========================================
+ Hits         3285     3297      +12     
  Misses        164      164              

Impacted Files	Coverage Δ
skll/config/utils.py	96.15% <80.00%> (-0.08%)	⬇️
skll/experiments/__init__.py	94.63% <100.00%> (ø)
skll/experiments/input.py	100.00% <100.00%> (ø)
skll/experiments/output.py	97.44% <100.00%> (+0.02%)	⬆️
skll/experiments/utils.py	93.38% <100.00%> (+0.22%)	⬆️
skll/types.py	100.00% <100.00%> (ø)

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[desilinguist]

This is part of #561.