add slottable behavior

[keithamus]

This adds a new @slot decorator for querying/manipulating/listening for slot elements.

This also listens for slotchanged events and calls the setter which can be very useful for running side effects on a slot change.

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[colebemis]

I don't feel qualified to review this since I don't have much web component experience. I'll defer to @koddsson for review.

[jonrohan]

Nice functionality 👍🏻