[Data Liberation] Recognize self-closing blocks in WP_Block_Markup_Processor (#2120)

Adds a is_self_closing_block() method to WP_Block_Markup_Processor to
enable detection and rewriting of block comments such as

```html
<!-- wp:core/separator /-->
```

This will be needed in the markdown processor.

A part of #1894.

 ## Testing instructions

CI. See the unit tests updated in this PR.

Author: adamziel