[PP-1] Add entity_browser widget for selecting existing Anchor Blocks

Currently the entity_embed button via entity_browser only allows to add new Anchor blocks. In some environments and on multilingual projects it's useful to be able to select existing Anchor Blocks. Anyway we'd have to create a Entity Browser view for this in the current situation, which seems too heavy, see: ✨ Add simple "Autocomplete" Widget selector plugin for entities Closed: duplicate

Let's hope that will one day be fixed so we can include a simple solution here.