Improve focus-trapping robustness

[jelbourn]

The current <focus-trap> directive uses a very simple (naive) approach to focus trapping.

• It assumes that the tab order is the same as DOM order, which is not necessarily true.
• Things like tabIndex > 0, flex order, and shadow roots can cause to two to misalign.

This behavior needs to be improved, using ally.js as a guide.

This is somewhat complicated by the need to detect different browsers / platforms, for which behavior varies subtly.

[jelbourn]

The work I had in mind for this issue is done

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