Fix 'core/interface' registration error

[gigitux]

Fixes #723.

This PR fixes the core/interface registration error, but it doesn't fix the other errors mentioned in the issue.

This error happens because core/interface is a package included in the bundle and not available on the document object.
Currently, on the /new-pattern/ route are loaded the site-editor scripts and the bundle of the pattern creator editor.
Both of them, under the hood, load the @wordpress/editor package and this causes that the core/interface store is registered twice.

With this PR, the @wordpress/editor is bundled in the pattern creator editor and the site-editor scripts aren't loaded. In this way, the core/interface store is registered once.

At the same time, while this PR can be a good temporary solution, I think that we need to explore #719. As mentioned in #719, this will reduce the maintenance cost and fix other issues too.

Props ,

Screenshots

How to test the changes in this Pull Request:

1. Visit /new-pattern/
2. Open the dev tools.
3. Ensure that Store "core/interface" is already registered. isn't visible.

[ryelle]

Looks good, fixes the error as described.

The failing test is also failing on trunk, and it's two parts— first, the call to npm run test:unit --workspace=wporg-pattern-directory-theme can be removed, since the old theme is not used anymore. But then the creator tests also fail with "SyntaxError: Unexpected token 'export'". Out of scope for this PR, and maybe irrelevant with #719 since it removes the separate creator plugin.

[gigitux]

Thanks for the review! Should I merge the PR?

@ryelle let me know how I can assist you to make #719 ready to be reviewed!