Add Support for Reusable Blocks

[SiobhyB]

Reusable blocks allow users to quickly re-use blocks, without needing to remember specific formatting or settings. This can be a great time-saver. Mobile users especially may appreciate being able to quickly select a block that they know is formatted in a way that looks good across devices. Reusable blocks aren't currently accessible from the app, however. It'd be great to add them!

This originally came up in a one-star app review from a user who seems to heavily rely on reusable blocks when setting up their posts/pages:

Pretty useless for me. All of my pop ups...reusable blocks etc are unsupported.

Android version here: https://github.com/wordpress-mobile/WordPress-Android/issues/11351

[hypest]

Closing in favor of the cross-platform ticket wordpress-mobile/gutenberg-mobile#2497.