Create metapackage of (loose) contrib test dependencies

Problem/Motivation

Over the years drupal/core-dev has been (ab)used as a requirement for running phpunit tests to setup Contrib testing.

This package is generally perfectly synchronized with core which is great for when you need to test drupal/core, however it is not necessarily as great when your testing contrib.

#3526142: Update PHPStan to 2.1.17 is an example where Core bumped itself up on a dev dependency that will be likely to impact contrib. Unlike Phpunit where Core itself needs to support the running version of PHPUnit in its TestBase classes PHPStan is generally run independently. PHPStan has it own plugin ecosystem and each contrib project may use it differently. Core does not need to (generally) support PHPStan ^1 for contrib to still benefit from it and vice versa before Core supported PHPStan ^2 contrib could have benefited from being able to upgrade and test without core-dev preventing it.

We likely still want a method to make it easy to setup a dev lab to run tests for contrib. A separate meta package that includes the common items, loosely constrained would allow contrib to easily set all dependencies without having to manual track core.

Some existing entries such as phpstan/phpstan or micheh/phpcs-gitlab may not be even needed in this package as they are mostly present for Drupal Core itself rather than contrib.

Note: I do not believe I am the first to suggest this option, I believe this was mentioned by at least one other community member in passing on Slack although I can not recall who or when it may have been.

Steps to reproduce

Proposed resolution

Create a new meta package that will make it easy for contrib to setup a testing environment, however not be explicitly tied to Drupal Core internal logic.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet