[webcomponents] isEncapsulated naming (#107)

https://github.com/w3c/webcomponents/commit/adae25b558edce3aaa31445abd4b44dc2e7b07a7 introduced the isEncapsulated flag to solve #61. It seems pretty great!

However, the naming is a bit weird.

- No other booleans on the platform that I know of use an "is" prefix. And, Event itself definitely does not: `bubbles` and `cancelable` are the models to follow here. So, `encapsulated` makes more sense.
- The term "encapsulation" is a new one that doesn't appear in other shadow-related APIs, as far as I know. Maybe there is something better? I cannot think of anything that is not overly verbose though (like "stopsAtShadowBoundaries"). But I thought others might have opinions, e.g. @annevk.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/107