Add a way to control spacing between gallery images

[mtias]

It'd be great to allow changing the size of the gaps between images when using galleries.

By default:

With zero gaps:

Once it's absorbed into the style system, this would also be an attribute the theme should control.

[tdechangy]

This should certainly be a WP/theme option first.

[andrewserong]

Just linked a PR, this issue will be addressed by #34608