What problem does this address?

Parallel to efforts in WordPress core, we should consider implementing PHPStan Analysis on the Gutenberg codebase.

Based on the success the Performance team has seen with implementing PHPStan, I would recommend we implement PHPStan regardless (and even to a stricter extent) of what makes it into WordPress core.

What is your proposed solution?

1. Add PHPStan analysis to the codebase.
2. Baseline all existing errors to a TDB level, and enforce that level on new code with CI
3. Independently review and remediate uncovered errors / preexisting tech debt.