Fix using the classic block in nested contexts

[youknowriad]

At the root level, the classic block omits its delimiters in order to support posts written with alternative editors. In nested contexts though, the classic editor should be rendered with delimiters otherwise the parser won't be able to reparse the post properly.

closes #13187

Testing instructions

• Use a classic block in a columns block
• Save and refresh
• Everything should work as intended.

[aduth]

We should document these options. I think the "best" way would to define a JSDoc typedef of something like WPBlockSerializationOptions with each property described. Unfortunately docgen won't yet output these, but it seems prime for a future enhancement.

[aduth]

Nit: I think we should try to consolidate our vocabulary around "inner blocks". Do we have many other references of "nested" in code? I know there's a tendency to refer to them this way in casual conversation. Is isInnerBlock reasonable as an alternative here?

[aduth]

At this point, I think we'd just not want to bother with a switch and use if instead. Alternatively, we could intentionally use fallthrough to allow the nested case to fall through to default.

switch ( blockName ) {
	case getUnregisteredTypeHandlerName():
		return saveContent;

	case getFreeformContentHandlerName():
		if ( ! isNested ) {
			return saveContent;
		}

	default: {
		const blockType = getBlockType( blockName );
		const saveAttributes = getCommentAttributes( blockType, block.attributes );
		return getCommentDelimitedContent( blockName, saveAttributes, saveContent );
	}
}

It's pretty confusing though. Maybe less-so if default was just code following the switch instead of a case, but still confusing I think.

[aduth]

Pretty simple fix 👍

[aduth]

Should be singular isInnerBlock?

Edit: Oh, I guess in the context that this is an option just passed along from serialize which is for multiple blocks, it makes sense as written, unless we wanted to rename the option as it applies to the context of the singular block being serialized. I think that comes with its own confusion. I'll leave it to you to judge, but I'm fine either way.

[youknowriad]

I think I'd leave it as is.