Introduce basic wp-env environment

[t-hamano]

Currently, I think either of the following methods is mainly used to develop this project locally.

• Set up your own local environment and clone this repository in the plugins directory
• Use the globally installed env package, and define a path relative to this project in the plugins property of .wp-env.json.

I don't think the introduction of wp-env has been discussed in this project yet, but it would be useful to be able to set up a local environment with only the npm packages and commands that this project depends on, without having to prepare a special environment.

This PR introduces a basic wp-env execution environment.

• run npm ci
• run npm run wp-env start
• Access localhost:8888/wp-admin and confirm that the Create Block Theme plugin works.

[vcanales]

Thanks, I agree that this is missing! This is good to go as is.