FSE: Scaffold editor-domain-picker package

[sirreal]

Changes proposed in this Pull Request

Scaffold a "editor-domain-picker" package:

• package scripts for building
• PHP enqueuing
• "Feature flag" (A8C_FSE_DOMAIN_PICKER_ENABLE)
• Some friendly placeholder JS to show the bundling is working.

Testing instructions

Apply D44014-code

Try with the following conditions (add to sandbox):

• Nothing special defined (feature disabled)
• define( 'A8C_FSE_DOMAIN_PICKER_ENABLE', false ); (feature disabled)
• define( 'A8C_FSE_DOMAIN_PICKER_ENABLE', true ); (feature enabled)

1. Sandbox a simple site and load the editor: you should see the friendly debugging messages on the console if the constant enables the feature.
2. When not enabled, the script should not be enqueued, check for it in the network tab. editor-domain-picker and shared dependency common-data-stores.
3. For other pages (not the block editor) nothing should be enqueued or run. Verify same behavior as when the feature is disabled.

No new/related PHP errors or warnings should be generated under any of the above conditions.

[matticbot]

Test live: https://calypso.live/?branch=add/fse-domain-picker

[matticbot]

Caution: This PR affects files in the FSE Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D44014-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing in the FSE Plugin for more info: PCYsg-ly5-p2

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[alshakero]

Reviewed the code and tested as instructed. All LGTM.