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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService) | Coverage Δ | | |---|---|---| | [skll/config/utils.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9jb25maWcvdXRpbHMucHk=) | `96.15% <80.00%> (-0.08%)` | :arrow_down: | | [skll/experiments/\_\_init\_\_.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy9fX2luaXRfXy5weQ==) | `94.63% <100.00%> (ø)` | | | [skll/experiments/input.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy9pbnB1dC5weQ==) | `100.00% <100.00%> (ø)` | | | [skll/experiments/output.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy9vdXRwdXQucHk=) | `97.44% <100.00%> (+0.02%)` | :arrow_up: | | [skll/experiments/utils.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy91dGlscy5weQ==) | `93.38% <100.00%> (+0.22%)` | :arrow_up: | | [skll/types.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/732?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC90eXBlcy5weQ==) | `100.00% <100.00%> (ø)` | |

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[desilinguist]

This is part of #561.