Allow adding E2E test suite

[swissspidy]

Feature Request

In https://core.trac.wordpress.org/changeset/45570, a new E2E test setup was added to WordPress core.

Maybe we can provide a similar setup for the plugin scaffolding so that developers can easily write E2E tests themselves.

Not sure how feasible that would be.

[nerrad]

One of the tricky things here is navigating what happens when plugins might have their own package.json file as part of implementing this will be adding a dependency to the @wordpress/scripts package (for exposing wp-scripts test-e2e.

[schlessera]

I think we can skip creating the package.json file then and show a warning with the npm command to run to adapt the file.