Add a simple widget area in footer.

[kjellr]

... to make use of Gutenberg 5.8's experimental widget feature.

Currently, if you load the widget screen while this theme is active, you can't test it out:

This PR creates a barebones widget area on the bottom of the page, so that this feature can be tested without switching themes:

[kjellr]

Closing due to inactivity. 🕐