Add code linter and coding standard check

- [ ] Add instructions to README
- [ ] Link to instructions from CONTRIBUTING.md
- [ ] Run linter as an independent GitHub Action (with a `pre-commit` GitHub Hook)
- [ ] Linter should also verify compliance with the coding standards
- [ ] Linter should have good integration with VS Code

Starting points:

- built-in linter (`php -l `)
- https://web20university.com/posts/php-linting/
- https://code-basics.com/languages/php/lessons/linting
- https://psalm.dev/

This is a follow-up to #3 where we require contributors to make sure code lints.