feat(a11y): initial interactivity checker

[jelbourn]

R: @hansl

This is the first step towards adding a general solution focus trapping, with the first goal of supporting MdDialog. I drew a lot of knowledge from ally.js.

Current checks are basic compared to the robustness of ally.js, but should capture most common use cases.

[hansl]

Do you want to export that out of core yet?

[jelbourn]

I'll add an export when I start using it elsewhere. Looks like there's a legit failure on IE11 and Edge, though.

[jelbourn]

@hansl Fixed IE and Edge problem, any further comments?

[hansl]

LGTM.

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