Add width to setting and styles

[oandregal]

Part of #20331

There's a use case for themes to be able to set the width of the site logo block. See #30907 (comment)

This value is used both as a setting for the block and a style. For themes to set these values via theme.json we need to:

• add a width setting blocks can use via useEditorFeature
• add a width style that will be appended to the stylesheet generated by global styles

As for the group for these values to live, it sounds like spacing could be a fine fit.