Reusable blocks have moved to a multi entity save flow

A change in Gutenberg 9.7 has meant that inline editing of reusable blocks is now possible. Changes are saved outside of the normal saving, and are part of a 'multi entity save flow'.

The P2 Editor needs to be aware of this and probably also perform what is needed for this multi entity save flow.

https://github.com/WordPress/gutenberg/pull/27887

See Automattic/p2editor#507

Start by reviewing Gutenberg PR 27887 and the related Automattic/p2editor#507 issue, then inspect how the P2 Editor handles saving reusable blocks. The work is done when inline reusable-block edits participate correctly in the multi-entity save flow.

Written by the indexing model from the issue text.