This adds support for the experimental `invoketarget` and `invokeaction`
attributes, as specified in the open-ui "Invokers" explainer.
(https://open-ui.org/components/invokers.explainer/)
The `invoketarget` attribute maps to the IDL `invokeTargetElement`,
similar to `popoverTargetElement`, and the `invokeaction` is a freeform
string.
The Button behaviour checks for `invokeTargetElement` in its activation
behaviour, and dispatches an `InvokeEvent` if there is one.
This also adds some basic scaffolding for `handleInvokeInternal` which
will allow elements to provide their own invocation action algorithms.
Differential Revision: https://phabricator.services.mozilla.com/D190449
c9c6527520