Parser: Add "void" block signature

[dmsnell]

Resolves #1085

Sometimes a block doesn't need anything inside of the block comment tags
as children. One example of this is the traditional "more tag," which
only needs to indicate position.

These changes introduce a new syntax element, the "void block,"
borrowing from the HTML5 nomenclature for HTML void elements.

In this new element, there are no children.

Since nothing changes on the JavaScript size this could be considered an
optimization whereby any block with no children serializes to a
self-closing block comment tag, saving some space and improving
readability of the post_content.

[westonruter]

@dmsnell is this not fixing #1085?

[mtias]

I like this for the terseness, thanks for the quick addition.

[nylen]

The syntax and code seem fine 👍

This PR should also update the serializer to use this format if a block has no content, and one or more tests should be added.

[dmsnell]

I'm having real trouble here making the tests work for this and I'm also experiencing an odd problem on save.

Problem with tests

• The tests in full-content.js are hard-coded to a behavior I don't understand. it looks like those tests are supposed to be snapshot tests but they also appear to be testing a few different types of behaviors and I can't figure out what. there's quite a bit of programming in that test file which is making it hard for me to understand the goals.

Problem with save behavior

• We seem to be exhibiting a behavior on save where if we don't have a block of a specific type registered that it will transform that block into a freeform block. This isn't good because the block could be a valid block and we may just happen to not have the right plugin installed. In the editor view I can understand treating it as freeform (though that's probably wrong too since it's a valid block) but we shouldn't be mangling it on save. I think this is an error not a decision, or at least I hope so.

Not sure how to move forward with this until I can get some help on these tests. Manual testing seems to work as expected.

[nylen]

5ebe561 fixes the tests. This change really should have been part of #947 (and tests for the bugs fixed there should have been added there).

The intent and functionality of these tests are documented here.

Most of the test fixture changes necessitated by using the PEG parser look OK, one that I'm not sure about is adding \n as a child received by multi-paragraph text blocks.

[dmsnell]

Thanks for the fixes and the link to the explanation @nylen!

This change really should have been part of #947

I'm not sure I follow why. Did the TinyMCE parser do this already? I hadn't realized it was a goal of the grammar before a few days ago.

Anyway, I am grateful for your help!

[nylen]

By "this change" I mean the specific change to use the PEG parser in the test suite as well.