Add support for scriptModule and viewScriptModule assets

[fabiankaegy]

Related Issue/RFC: #368

Description of the Change

Add support for scriptModule and viewScriptModule assets defined in block.json files.

These assets are handled by a separate instance of Webpack which is configured to output JS Modules instead of commonjs bundles.

In order to make this work we needed to split up the Webpack setup into two separate instances. One side-effect of this is that we can no longer run clean during watch mode since it would clean assets from the other webpack instance and therefore cause unexpected issues.

Important

We need to determine what impact this PR has on how easy it is to extend toolkit with a custom Webpack config. Because we are changing the interface that we export from an object to now an array of objects.

Alternate Designs

I can't think of alternative designs for this.

Possible Drawbacks

The extensibility needs to be figured out. Also with two separate webpack instances this may be a little slower / use more resources.

Verification Process

The bundled example block in the 10up-theme project has both a regular script and also a view module which uses the interactivity API.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.
• I have added a changeset to my PR. See CONTRIBUTING document for instructions

[changeset-bot]

🦋 Changeset detected

Latest commit: fac2194

The changes in this PR will be included in the next version bump.

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[nicholasio]

LGTM