[A11Y] Add `role="main"` attribute to primary content region in each Layout template

Description of the need

This is a follow-up to https://github.com/backdrop/backdrop-issues/issues/5600

It would be nice to have a follow-up to recreate the <main> tag. Seems like there should definitely be one on all pages, even if they're just administrative pages. https://github.com/backdrop/backdrop-issues/issues/5600#issuecomment-111...

The lack of a <main> tag came up in that issue and @quicksketch asked for this follow-up issue.

At first, I thought that this was specific to the admin theme, but I'm not seeing any <main> tag in our front end themes/layouts either. I would like some help understanding why this is important, if no one has asked for it before?

I assume that to fix this, we'd just need to add the main tag to all our layouts. It would seem to me that we'd have some issues with Backward compatibility and/or contrib themes that override layouts.

@gitressa - any thoughts?