When switching to a block theme, classic sidebars were orphaned and their widgets remapping to the `'wp_inactive_widgets'` sidebar . This changeset preserves the sidebars and their widgets, providing a migration path to a block theme without losing the widgets.
Classic sidebars are now:
* Stored in a new theme mod called `'wp_classic_sidebars'`;
* Restored to the `$wp_registered_sidebars` global variable when the `'widgets_init'` action fires (via a new internal function called `_wp_block_theme_register_classic_sidebars()`);
* And marked as `'inactive'` when interacting with sidebars REST API endpoint.
References:
* [https://github.com/WordPress/gutenberg/pull/45509 Gutenberg PR 45509] which adds an option for importing widgets from sidebars into template parts.
Follow-up to [50995], [6334].
Props mamaduka, audrasjb, hellofromTonya, ironprogrammer, jameskoster, joen, matveb, mukesh27, noisysocks, poena, youknowriad.
Fixes#57531.
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