Switch to @wordpress/env for development and testing

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

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

Start by reading integrations.yml and bin/install-wp-test.sh, then review the existing development and testing documentation. Try the proposed `wp-env start --xdebug=coverage` and test command to understand the workflow. Done means adding .wp-env.json, updating integrations.yml, removing the SVN setup and bin/install-wp-test.sh, and updating the documentation.

Written by the indexing model from the issue text.