Linting tools: add PHPStan

[jeherve]

Changes proposed in this Pull Request:

- Linting: add PHPStan tools to analyze the codebase

• Avoid issues with custom apply_filters
• Fix level 4 issues
• Add PHPStan to linting action

A few notes:

• When trying to use PHPStan's options to ignore specific lines, I kept getting memory fatal errors. So I had to make those ignore rules in the config file instead.
• When trying to up the config to level 5, I keep running into the same memory Fatals locally. I may try it in the CI though.

Edit: I see I get the same fatals in the CI already, even at level 4:

PHP Fatal error:  Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in phar:///home/runner/work/posts-on-this-day/posts-on-this-day/vendor/phpstan/phpstan/phpstan.phar/vendor/nikic/php-parser/lib/PhpParser/Lexer.php on line 263
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in phar:///home/runner/work/posts-on-this-day/posts-on-this-day/vendor/phpstan/phpstan/phpstan.phar/vendor/nikic/php-parser/lib/PhpParser/Lexer.php on line 263
Script ./vendor/bin/phpstan analyze -c phpstan.neon.dist --memory-limit 128M handling the phpstan event returned with error code 255

Testing instructions:

• Do the tests pass?