Consistent phpunit runners for local and CI

[kasparsd]

Development workflow consistency:

• Standardizes local development and CI around wp-env so both paths use the same environment bootstrapping and runtime logic.
• Adds a small npm command layer for common tasks like CLI access, linting, formatting, PHPUnit, and coverage, so developers and CI invoke the same commands.
• Updates the coding standards workflow to run through the wp-env-based toolchain instead of relying on a separate runner-level PHP setup.

Local setup and onboarding:

• Commits a Node version via .nvmrc and upgrades @wordpress/env, reducing local setup drift and keeping machines closer to CI.
• Expands the contributing docs to make wp-env the default local workflow, covering start/log/stop commands, PHPUnit usage, coverage, and version overrides.
• Supports both .wp-env.override.json and WP_ENV_PHP_VERSION / WP_ENV_CORE env vars, making local environment customization straightforward without changing repo defaults.

Local PHPUnit execution:

• Moves PHPUnit execution into the wp-env tests container, removing the old split between custom local setup and CI-only test installation.
• Adds a dedicated tests:cli path and install flow for test dependencies, so local test runs are easier to reason about and reuse.
• Aligns the single-site and multisite PHPUnit config files with this workflow, including cleaner cache and coverage paths.

WordPress and PHP version matrix support:

• Makes it possible to run PHPUnit locally against different WordPress core and PHP combinations by driving wp-env from override files or environment variables.
• Broadens Composer dev tooling to support multiple PHPUnit versions and include the WP-CLI bundle needed for this workflow.
• Replaces the hardcoded GitHub Actions exclusion list with a generated PHP/WordPress compatibility matrix, centralizing supported combinations in one script.

CI parity with local testing:

• Uses the generated matrix in GitHub Actions so each job boots the requested WordPress and PHP combination the same way it can be done locally.
• Refreshes Composer dependencies per matrix job so the right PHPUnit version is installed for that exact WordPress/PHP pairing.
• Extends the same wp-env-based approach to coverage runs, improving parity between local verification and CI.

[github-actions]

🧪 Test this PR on Playground

[kasparsd]

The remaining parts of this are now included in fairpm#493