Fix theme.json styles for the core/list block

[oandregal]

Alternative to #32273

I was trying to add some styles for the core/list block via theme.json, using something like:

{
  "version": 1,
  "styles": {
    "blocks": {
      "core/list": {
        "color": {
          "text": "hotpink"
        }
      }
    }
  }
}

I expected the core/list block to have those styles applied in both the editor and the front end. What happened instead was that it worked in the editor but not in the front end.

Why it fails

In the editor, the block gets a few extra classes attached, including the wp-block-list. However, in the front, it doesn't get any class attached.

The stylesheet generated from theme.json taking into account the block.json of a block to know what selector should use for a particular block. If the block.json contains a __experimentalSelector key, that's what it uses. Otherwise, it uses the block class selector. Because the core/list block doesn't declare any __experimentalSelector, the wp-block-list` is used, resulting in the styles not being applied because the block markup doesn't have that class.

How this fixes it

This PR fixes it by declaring the block selector for the core/list block that "global styles" can use to output the stylesheet.

Other alternatives considered:

• Add the .wp-block-list class to the block. Discarded because this is not backward compatible: existing blocks won't work because they don't have the class applied. This is what is done at List Block: Add typography styles to theme.json #32273

[youknowriad]

Makes sense.