Partially addresses #8699.
This adjusts the Stimulus toggle controller so you can toggle content in both directions via a single element. This is in addition to the previous behaviour for toggling via multiple elements like radio buttons when each element always toggles in one direction only.
If a toggle element contains a chevron icon this will automatically toggle the direction of that icon too.
Note, in order to not have to re-implement the animation provided by the slideToggle() function in standard JavaScript, this just switches the style :display between 'none' and 'block' so it is not as smooth. Perhaps it could be made more smooth later with a CSS transition.
