Blocks: Automatically generate a wrapper classname for blocks

[youknowriad]

try to close #1362

Glitches:

• Doesn't work for blocks with save function returning strings (like embed links)
• Doesn't work for block with save function returning a component (instead of an element) but IMO we should drop this because we don't need a Component in the save function (we don't need any local state, ref or something it's just a serialization)

Notes:

We may need the same thing on the backend for dynamically rendered blocks but it's harder to implement because we only have strings (and not element) in the backend.

[mtias]

Can we drop core from the classes?

[youknowriad]

What about plugin blocks with other namespaces? this could create ambiguity.

[mtias]

How so? Core blocks would have no namespace and plugin blocks will have the plugin namespace.

[mtias]

It'd be nice to have a list of blocks where we don't want to apply the class (p and heading, to start).

[youknowriad]

what about a block setting className that can be nullable and if not set we default to wp-block-core-quote

[nylen]

@youknowriad why do we drop the core/ namespace here? I think it would be good to have a direct correspondence between the class name and the full block name including namespace.

Alternative suggestion: wp-block__core__text

From docs/design.md:

Key Values
All blocks are created equal, ...

[youknowriad]

We had a discussion in Slack (should have reported this here), and we decided to drop the namespace for core blocks to shorten the classname (and we'll forbid the core namespace for third party blocks). cc @mtias

[nylen]

I think we should revisit this decision. It's a definitive way in which core blocks are not equal to plugin blocks; it imposes additional cognitive load; and anything related to how we handle block names should be super simple, consistent, and explicit.

This breaks down a bit if, for example, a plugin registers a block named text/poetry. Then you end up with classes wp-block-text for core/text and wp-block-text-poetry for text/poetry.