refactor: split phpstan-baseline into smaller files

[paulbalandan]

Description
This PR splits our existing enormous phpstan-baseline.php into smaller baseline files named by their identifiers, thanks to https://github.com/shipmonk-rnd/phpstan-baseline-per-identifier/tree/2.0.0. Currently, the baseline contains 3,741 errors. That's very overwhelming to fix. TBH, when looking at the baseline I cannot continue further because of the bulk of technical debt we have.

The pro of this approach is that we can attack the baseline by identifier, making the cleanup less overwhelming. The cleanup would be bite-sized. The con of this approach is that we have too many baselines now, but that is the consequence of having 3.7k+ errors.

Same command to generate the baseline: composer phpstan:baseline

P.S I have fixed some of the files as PHPStan complains of No error to ignore stuff.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide