Make auto-generated wp-block* classes consistent

[nylen]

Follow-up to #1381 that modifies the generated classnames for blocks to improve consistency.

Mainly I don't think we should drop the core namespace from generated classnames for core blocks. From docs/design.md:

Key Values
All blocks are created equal, ...

From #1381 (comment):

This special handling is 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.

Also, the current structure 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.

The last bit is probably more an issue if someone releases a plugin whose name overlaps with one of our embeds, then the class names will get a bit confusing. We can fix this by making it really obvious how a block name maps to a class name, and making the different parts of the class name really clear.

Here are some example class names before and after this PR:

Block name	Before	After
core/text	wp-block-text	wp-block__core__text
core/image	wp-block-image	wp-block__core__image
core-embed/collegehumor	wp-block-core-embed-collegehumor	wp-block__core-embed__collegehumor
myplugin/someblock	wp-block-myplugin-someblock	wp-block__myplugin__someblock

As a first try, I've used __ because then the parts of the classname are completely unambiguous.

[mtias]

This is a bit different than namespacing on registration. I don't think "core" is providing any semantic value to the classes, mostly noise to most people styling things, which should be the primary concern here. The example of "text/poetry" doesn't seem great because such a namespace would have issues in many areas (PHP functions being text_*).

[nylen]

There are still a couple of things to fix here:

• wp-block-core-embed-collegehumor - we should remove core here as well to be consistent
• We should ensure that this generated classname appears first in the serialized markup

[nylen]

Previously we were using kebabCase here which changes strings such as table2 to table-2. This is not desirable. This PR also contains the fix + tests:

-<table class="wp-block-table-2">
+<table class="wp-block-table2">

[nylen]

The changes here are now much less extensive:

Block name	Before	After
core/table2	wp-block-table-2	wp-block-table2
core/image	wp-block-image	(no change)
core-embed/collegehumor	wp-block-core-embed-collegehumor	wp-block-embed-collegehumor
myplugin/someblock	wp-block-myplugin-someblock	(no change)

Merging to address the inconsistencies in #1599 (comment) and the bug in #1599 (comment).