⚛️ Remove the wp-inner-blocks wrapper

[DAreRodz]

We need to get rid of the <wp-inner-blocks> element to avoid CSS issues like #50.

As @luisherranz proposed in #50 (comment), we can append the wp-inner-block attribute to those blocks that are the children of an interactive block.

<div class="wp-block-parent">
  <div>Some content</div>
  <div class="wp-block-child-1" wp-inner-block>Some child content</div>
  <div class="wp-block-child-2" wp-inner-block>More child content</div>
</div>

Note that we need a new attribute because:

• using a regexp to find nodes with their class starting with wp-block-" could be not reliable enough
• using the wp-block-type attribute would work only if children are interactive blocks as well

[luisherranz]

Fixed in #77.