Allow esc to close the / menu #5

Currently if you hit / to open the block menu (BlockCommandBar), the only way to exit is to either select a block, or hit delete.

This is a little annoying, especially if you hit the + button (i.e. not entering the / char).

It should also work to just hit esc to remove the / (and anything typed after it) and close the menu. Ditto clicking / tapping outside the menu.

We should probably make BlockCommandBar take a proper input (not just use text block input) but this works for now.