Switch to @wordpress/env for development and testing

[GaryJones]

Requires Docker, but allows us to streamline development and testing in other ways.

• Add a .wp-env.json file
• Install npm packages during integrations.yml
• Remove need for installing SVN during integrations.yml
• Remove bin/install-wp-test.sh
• Update documentation

# Start with:
wp-env start --xdebug=coverage
# Run tests with:
wp-env run tests-cli --env-cwd=wp-content/plugins/zoninator composer test