Feature: Block-based template parts for Classic themes

[ockham]

Fork of @Mamaduka's #3234, to apply this small fix.

Quoting that PR's description:

Testing Instructions

Classic theme - TT1

• The new "Template Parts" menu shouldn't appear in Appearance.
• Users can't access the /wp-admin/site-editor.php path directly.

Block theme - TT2

• The new "Template Parts" menu shouldn't appear in Appearance.
• The "Editor" menu is accessible.
• The Site Editor works as before.

Themes with block-based templates part enabled You can grab the testing theme from this repo - https://github.com/Mamaduka/block-fragments

• The new "Template Parts" menu is visible under Appearance.
• The menu redirects to the "Template Parts" list view.
• The "Site" and "Templates" navigation items aren't visible.

Warning
The last item will require the latest package update PR to be merged, and it might not be possible to test immediately.

For any other details, please see the original PR.

(@Mamaduka is AFK and gave me permission to fork his PR via DM.)

Gutenberg tracking issue: WordPress/gutenberg#43440
Trac ticket: https://core.trac.wordpress.org/ticket/56467

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[ockham]

@fabiankaegy Would you mind approving this PR? 😊

[fabiankaegy]

Thanks for updating the PR 🚀

[fabiankaegy]

@fabiankaegy Would you mind approving this PR? 😊

I'm not sure whether I have sufficient access to actually approve it. But from my perspective this is ready to go :)

[hellofromtonya]

Retested per my Test Report. Confirmed @fabiankaegy resolves the white screen issue of attempting to access /wp-admin/site-editor.php ✅

Testing Report

Env:

• WordPress: trunk
• Plugins: none
• Browser: Firefox
• Localhost: wp-env

Results:

Classic theme - TT1, TT0

• The new "Template Parts" menu shouldn't appear in Appearance ✅
• Users can't access the /wp-admin/site-editor.php path directly ✅

Block theme - TT2, Frost

• The new "Template Parts" menu shouldn't appear in Appearance ✅
• The "Editor" menu is accessible ✅
• The Site Editor works as before ✅

Themes with block-based templates part enabled
Theme: https://github.com/Mamaduka/block-fragments

• The new "Template Parts" menu is visible under Appearance ✅
• The menu redirects to the "Template Parts" list view ✅
• The "Site" and "Templates" navigation items aren't visible ❌ (needs the JS packages updated - per notes in the description)

Observations:

With the block-fragments (classic theme with block-based templates) enabled, I am able to directly access /wp-admin/site-editor.php but it's a white screen. No errors in the browser's console or error logs.

Fixed ✅ Now when attempting to go directly to /wp-admin/site-editor.php, the user gets an error as expected:

The theme you are currently using is not compatible with the Site Editor.

[hellofromtonya]

• The PR works as expected, minus the one test point that is dependent upon package updates (i.e. which will be retested once updates are backported).
• The PR meets Core's coding standards.

Prepping the commit

Observation:
There are a few optimizations to explore, e.g. instead of invoking current_theme_supports() et al repeatedly within the same file. However these are out-of-scope for this PR.

[hellofromtonya]

Thank you everyone for your contributions! Committed via https://core.trac.wordpress.org/ticket/56467.