MU WPCOM: Port the newspack blocks from the ETK (2nd try)

[arthur791004]

Related to p1722871269505439-slack-CRWCHQGUB

Proposed changes:

• We merged MU WPCOM: Port the newspack blocks from the ETK #38454 before but it had a conflict with the newspack-blocks plugin on atomic sites, so the changes were reverted yesterday.
• Centralize the feature flags of the ETK plugin by the load_etk_features_flags function & Load the newspack blocks feature only when the Newspack_Blocks isn't declared- 2803837
• Load the ETK features as normal but limit the editor-related features to the editor pages or frontend pages to avoid fatal errors when activating the plugin - 6039568

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to your atomic site
• Install the newspack-blocks plugin via https://github.com/Automattic/newspack-blocks/releases/tag/v3.6.0
• Make sure the fatal error won't be shown

  PHP Fatal error:  Cannot declare class Newspack_Blocks, because the name is already in use in /wordpress/plugins/wpcomsh/5.2.0/vendor/automattic/jetpack-mu-wpcom/src/features/newspack-blocks/synced-newspack-blocks/class-newspack-blocks.php on line 11
  

[taipeicoder]

The changes LGTM, see the conversation in p1722993219555279-slack-CRWCHQGUB for a more in-depth review.

[ellatrix]

Why did you add these conditions? It should always be enqueue_block_editor_assets for block scripts that register the blocks. See #41055.