Block Bindings: Add support for the List item block

[saulyz]

What problem does this address?

WP 6.7.1 empowers the creation of custom binding sources and the binding for blocks Header, Paragraphs, Buttons and Images.
That covers a lot, but some common text pieces are missing. The list items.

What is your proposed solution?

The structure and features of List items (core/list-item) share lot's of common with Headers and Paragraphs. So it's probably not a big effort to make it work.

[SantosGuillamot]

As explained in this issue, opening block bindings to more blocks is challenging, and there are a few aspects to solve first. We could add support to specific blocks, creating hardcoded specific solutions, but I think it makes sense to address the original problems.

I've just created the iteration issue for 6.8, where I explain that working on those limitations is one of the main topics for this release. Once that is solved, it should be safer to add support for not only the list item block, but for other blocks as well.