Add wp-env After Setup Command

[ObliviousHarmony]

What?

This pull request will execute a new lifecycle script, afterSetup, whenever WordPress is configured. It will run on:

• wp-env start: Runs when the cache is invalidated and WordPress is configured.
• wp-env clean: Runs after the environment is cleaned.

Why?

By allowing users to run their own command after installation, they will be able to define setup scripts and the like that perform additional processing on the environment post-setup.

How?

We've added an afterSetup option that can be set at the root of a .wp-env.json file. This option can also be overridden using the WP_ENV_AFTER_SETUP environment variable.

Testing Instructions

1. Create (or edit) a .wp-env.override.json to include:

{
  "afterSetup": "echo \"Test\""
}

2. Run npm run env -- start --update and verify that Test is printed once.
3. Run npm run env -- start --update --no-execute-scripts and verify that Test is not printed.
4. Run WP_ENV_AFTER_SETUP="ls -la" npm run env -- start --update and verify that the Gutenberg directory is printed once.
5. Run npm run env -- clean and verify that Test is printed once.
6. Run npm run env -- clean --no-execute-scripts and verify that Test is not printed.

[noahtallen]

Just one or two minor comments, but this is looking pretty good! It also tests well.

[noahtallen]

Another tip is that instead of npm run env -- start --update, you can use npx wp-env start --update. npx finds the local code as well

[noahtallen]

Alright, this looks good to me! I have one non-blocking concern, which is that the sometimes-executed sometimes-not behavior might be confusing to end users. But I agree that the behavior in the PR makes the most sense given how wp-env start works.