CODEBASE: Enable new lint rules and fix all lint errors

[catloversg]

We don't enable plugin:@typescript-eslint/recommended-requiring-type-checking and plugin:@typescript-eslint/strict because there will be too many lint errors (973 errors at this point) if we do that. Snarling handled tons of them in #501. I'll continue fixing all these lint errors until we can enable those rules.

Because there are too many lint errors, I'll create a series of PR instead of a massive one. I'll close this issue after all those PRs are merged.

Reference: #474 and #501.