chore: upgrade dependencies and rules

Updates the coding standards library from Slevomat. As a result, the
`UnionTypeHintFormat` has been replaced with `DNFTypeHintFormat`.

ci: address zizmor warning

chore: upgrade GitHub Actions

Because of the Node 16 -> Node 20 migration.

Switch to `PHP_CodeSniffer` v3.8.0

Unfortunately, `quizlabs/php_codesniffer` is officially abandoned.
A fork has been created at `phpcsstandards/php_codesniffer`.

As such, we now require `PHP_CodeSniffer` v3.8.0 or later. This
version fixes a lot of things and also adds support for PHP 8.3.

The default PHP version for the action is now PHP 8.3. This does
not impact downstream dependencies of this action as long as they
properly configured the version constraints.

Allow multi-line array with single value

Add initial version of the GEWISPHP Coding Standards

These are heavily based on the Doctrine and Slevomat Coding Standards.

Also includes a GitHub Action that can be re-used across projects.