Seedlet Blocks: Remove theme attributes from block templates

[jeyip]

Changes proposed in this Pull Request:

Remove theme attributes from seedlet blocks block templates

Context:

This PR was created as a follow-up to WordPress/gutenberg#28088. See the aforementioned PR description and code changes for more context.

Discussion:

1. What this means for theme designers is that, when creating block templates, the theme attribute can be omitted from serialized wp_template_part blocks (like we've done in this PR). The logic for the Gutenberg full site editor will automatically add a theme attribute that matches the current, activated theme's stylesheet.
2. New sites created for full site editing will not load template parts correctly until Template Parts: Fix loading issue WordPress/gutenberg#28088 and Avoid using auto-drafts for theme templates and template parts WordPress/gutenberg#27910 are added to a release, but to be fair, template parts are already broken on new FSE sites Site Editor: template parts not loading on Horizon wp-calypso#48145.

Testing:

• Apply this PR
• Run Gutenberg locally (see instructions here)
• Install the seedlet-blocks theme by referencing it in .wp-env.override.json

    // .wp-env.override.json
  
    // Replace ~/work/theme-experiments/seedlet-blocks with 
    // the path to your locally installed seedlet-blocks theme
    {
         "mappings": {
             "wp-content/themes/seedlet-blocks": "~/work/themes/seedlet-blocks",
         }
    }```

• npx wp-env start --update
• Activate the seedlet-blocks theme
• Open the front end and the site editor. FSE templates and template parts should load without issue.
• Reinstall the seedlet-blocks theme in a subdirectory by modifying .wp-env.override.json

     // .wp-env.override.json
    
     // Add a test subdirectory in the mapping
     {
          "mappings": {
              "wp-content/themes/test/seedlet-blocks": "~/work/themes/seedlet-blocks",
          }
     }```

• npx wp-env start --update
• Activate seedlet-blocks.
• Open the front end and the site editor. FSE templates and template parts should still load without issue.

Related issue(s):

Automattic/wp-calypso#48145

[Addison-Stavlo]

This looks good to me! ✅

I just holding approval until I know themes folk get a chance to see this new format so we are all on the same page. 😁

[pbking]

Looks fine to me. Along with WordPress/theme-experiments#172 I advise against merging until the version of Gutenberg with WordPress/gutenberg#28088 ships.

[jeyip]

This makes sense to me 👍 We can revisit this once Gutenberg v9.9 is released

[vindl]

LGTM!

[jeyip]

Quick update:
The Gutenberg v9.9 release is being pushed back to align with the release of WordPress v5.7 beta 1. Both are slated for release on February 2nd.

[jeyip]

Merging this because the v9.9rc is available on Gutenberg edge sites now