Allows Classic / Hybrid themes to use block-based template parts without using complete block-based templates.
* Exposes the Site Editor's template parts UI
* Adds Appearance > "Template Parts" menu
* Enabled within the theme via adding a theme support for `'block-template-parts'`
{{{#!php
add_theme_support( 'block-template-parts' );
}}}
This is a backport from Gutenberg.[https://github.com/WordPress/gutenberg/pull/42729 See WordPress/gutenberg PR 42729].
Follow-up to [52330], [52069], [52178].
Props mamaduka, fabiankaegy, poena, scruffian, manfcarlo, bernhard-reiter, hellofromTonya.
See #56467.
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