Add wp-env support

[joemcgill]

This adds support for using the @wordpress/env local environment setup now that WordPress/gutenberg#26507 has been merged.

Added:

• A new package.json file, which adds @wordpress/env as a dependency and a script for running the environment. See docs for wp-env.
• A .wp-env.json file which configures wp-env for our project

[joemcgill]

Note for testing: even thought the PR for setting ports for WP_HOME has merged, it's still unreleased, so to test this locally you'll need to clone the latest version of https://github.com/WordPress/gutenberg and run the following:

# Run npm link from the gutenberg/packages/env directory.
cd {path-to}/gutenberg/packages/env
npm install && npm link

# Link the irving-dev dependency of wp-env
cd {path-to}/irving-dev/
npm link @wordpress/env

# Spin up the environment
npm run wp-env start

WordPress should now be installed and running at http://localhost:8888/wp-admin/.
You can check that the npm link worked correctly by visiting Options > General in your and check that the Site Address (URL) is set to http://localhost:3001.

[jameswburke]

🐝