Adhere to CONTRIBUTING "Type Annotations" rule

[Aurumaker72]

Use type annotations wherever automatic type inference fails (or produces unnecessarily complex results) and developers would otherwise be required to scan big chunks of code to manually infer a type.

Particularly dofile results should be type-annotated wherever possible.

[FramePerfection]

This implies a whole bunch of nonsense, because this fake "strong" typing is effectively just a means of communication between developers.
Lua itself will never care for this factor, nor should mupen ever care for it either.
As such, this issue can mean, from a bottom up perspective, either or any combination of:

• A contribution guideline similar to CONTRIBUTING.md, which asks contributors to follow the rules of the repository's proposed ruleset
• An automated environment to check for compliance with the repository's defined ruleset, such as a GitHub action that would fail whenever non-compliance is detected.