Releases for Layout Builder Block Presets

Initial alpha release.

Features:
- Save any configured Layout Builder block as a named preset via "Save as preset" button.
- Presets store the full block plugin ID and configuration.
- Insert saved presets into any Layout Builder section via "Insert preset" button on the section toolbar.
- Presets are config entities — fully exportable via drush config:export.
- Admin UI to manage (add/edit/delete) presets at /admin/config/content/lb-block-presets.
- Two permissions: administer lb block presets, use lb block presets.