[webcomponents]: Let's reach consensus on Shadow DOM

Folks,

I wrote a long email, replying to each point where I agreed/differed with
Ryosuke, and then deleted it, realizing I wasn't being productive.

So instead, I decided to start summarizing the contentious bits of the
current Shadow DOM spec:
https://github.com/w3c/webcomponents/wiki/Shadow-DOM:-Contentious-Bits

We are at a point where there are hard choices to be made. But with the 4+
history of the adventure, it's nearly impossible for everyone to recall or
catch up on discussions and relevant insight.

With this doc, I am hoping we'll get on the same page and make way.

:DG<