To reduce the composer package size.

Add tests/ to export-ignore

This repository is currently using version 34 of the Mediawiki coding
style phpcs rules. If the code aims to comply with Mediawiki style
guidelines, it makes sense that it continues to comply at current
versions of the rules.

Besides being simply different, the newer versions of the rules
introduce, for example,
MediaWiki.Usage.NullableType.ExplicitNullableTypes, which enforces
compliance with coming deprecation rules in PHP 8.4 concerning nullable
types. If the code does not remove the soon-to-be-deprecated form of
nullable type declarations, the library will no longer be usable in
Mediawiki for PHP 8.4 deployments.

Resolves DataValues#100

Bug: T379481

…-codesniffer-20241206

Upgrade mediawiki-codesniffer to latest version (45.0.0)

Test with modern PHP

 * Drop support for PHP 7.2, 7.3
 * Upgrade codesniffer rules to current `mediawiki/mediawiki-codesniffer` version (45.0.0)
 * Make nullable type parameter declarations explicit for compatibility with PHP 8.4
 * Start testing with modern versions of PHP (8.1-8.4)