Add aria-pressed to the editor toolbar toggle buttons.

[afercia]

Fixes #628

[nylen]

I think this should just be { control.isActive }.

[afercia]

Now that I look back at that, I think the original intent was to have true and false as strings, because HTML attributes are strings. See also the aria-expanded for the inserter toggle. Minor point though.

[nylen]

React should handle this for us. It's worth checking what it's actually doing in the dom though. Is the absence of an aria-pressed attribute the same as false?

[afercia]

Is the absence of an aria-pressed attribute the same as false?

No because the presence of the attribute (even when false) make screen readers announce the button as toggle button instead of just button.

[aduth]

Looks good 👍

[nylen]

Thanks for checking, @aduth, I made those last few comments while mobile.

It would be good to circle back to this later and add tests to make sure we're rendering the attribute appropriately (see #641 (comment)).

For now, +1 for merging.

[jasmussen]

I see approval here, so I'm merging.