Investigate Global Styles

While I agree with the principle of not including a lot of styling, there is one area we can dig in, that of global styles elements. Anything we can do here moves things into `theme.json` and presents a UI letting the look and style of this be configurable in the site editor and moving all the work off of both this block and the theme using it to WordPress core.

We'd need to identify what _elements_ are present that make sense so that we have a definition of what should be managed by global styles and what should be a part of a stylesheet on a site that uses this block

Start by auditing the tab block's existing styling and the global-styles boundary described in the issue, focusing on which elements could be represented in theme.json and exposed in the site editor. Document the elements that belong in global styles versus the consuming site's stylesheet, with a clear recommendation for each.

Written by the indexing model from the issue text.