Allow setting `short_name` option through the site-title core block

[carstingaxion]

Following the repeated requests for a UI of the short_name option from #210 , I think it could be helpful to add some additional UI to the existing site-title core block to set this option.

I would suggest a similiar approach, like we did for the site-icon-maskable setting on the the site-logo core block over here at #919

A user, using a block-theme, with the intention of changing the site-title will do that - propably - in the Site editor, so having all related options in one place directly on the block-level would be handy, I think. In this case there should be no need for a user to visit wp-admin/options-general.php, similar to #915 .

[westonruter]

Yes, this makes sense to me.