Site Customizer: Should offer font pairing suggestions

[blackjackkent]

In the Gutenboarding site creation flow, we offer font pairing suggestions for users to ensure they have heading and body fonts that work well together. This always struck me as very helpful (as for me personally, font selection is an abject mystery which also has a huge impact on the success or failure of a site's design.) We should offer similar info to a user using the Customize tools to update their site's design; in this panel, we currently only have independent select controls.

[kwight]

All themes could have a Fonts panel for consistency (Varia-based themes do not), only the ones that are edited in the block editor will link there instead.