feat(popover): add support for popover target element

[yinonov]

Closes #66

Note that this PR:

• Refactored popover triggers/invokers handling
• Had to deviate and extend from the spec exact flow due to the lack of properties reference (e.g.

If node has a popovertoggletarget attribute, then set idref to the value of node's popovertoggletarget attribute

doesn't mentioned property check for popoverToggleTargetElement

• introduced another triggers.ts file to extract payload of code from the main popover.ts
• popover.ts is still cumbersome and already hard to read
• should organize code more neatly
• should write more tests
• remove comments

[yinonov]

@keithamus @jgerigmeyer
please review, as far as functionality. code runs as expected.
I'm yet sure about the style of code, it seem to bloat rapidly. WDYT?

[yinonov]

should popoverTargetElement element check advance to supported popover invokers check?
https://github.com/yinonov/popover-polyfill/blob/6c7176645aa396d242fd819795d872bb9b47a6e0/src/popover.ts#L213-L215

[keithamus]

This is looking close to shippable IMO. A couple of points though:

[yinonov]

I think we're done @keithamus

[keithamus]

This is looking great! Excellent work!

I'll leave @jgerigmeyer or @sanajaved7 to give a second review and merge, due to the size of the change. But this is 👌 in my view.

[jgerigmeyer]

One minor spelling suggestion, but this looks great! Thanks for the contribution. 🎉

[jgerigmeyer]

Published in v0.0.9 🚀