[select] fix(Suggest2): get root node correctly in Shadow DOM

[kutensky]

Fixes #5461

Changes proposed in this pull request:

Changed detecting is input focused by comparing with input's rootNode rather than with window

[palantirtech]

Thanks for your interest in palantir/blueprint, @kutensky! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

nice, thanks @kutensky! I think we'll have to audit other places where we use document.activeElement. it's probably worth having a shared helper function to get the active element which we can use across various components -- I'll implement that in a follow-up PR.

[kutensky]

Thank you @adidahiya for such a quick response and for adding support on other components. That helps a lot.