Expand/collapse all: initial v4.0 commit of plugin

1. **Core:** Tests and loads the details summary polyfill.  More work is needed to get this polyfill working which I'll take care of this week.
1. **Expand/collapse all:** Initial commit of the expand/collapse plugin:
 1. User can now specify their own expand/collapse all link element.
 1. Type of link element is set with [data-type] attribute.
 1. Link elements can be restricted to only expanding/collapsing
details within a given parent using [data-parent] attribute.

@masterbee can you take a look at this and let me know if it's in line with what you were thinking for v4.0 plugins.