Add a hook for collapsible content

Add a hook for collapsible content

This might allow a script to intervene after the inital setup and to
process custom state like for instance; autocollapse, inner/outercollapse.

I also note that the hook is deliberately AFTER the changes to the DOM,
since hooks can be executed after they have been fired, so predictable
DOM is required, which is only after the makeCollapsible setup.

Expose expand/collapse/toggle functions using element data.

Bug: T72762
Change-Id: I099c533740bbe15a1d65c8821bad8bb8395f3806