API: add StyleSheet (#2268)
API: add StyleSheet (#2268)

* API: add StyleSheet

Sources:
1. CSS Object Model specification:
   https://drafts.csswg.org/cssom/#stylesheet
2. Chromium repository:
   https://github.com/chromium/chromium/blob/1182c84d9849940a6f6be3ec22d8b244dc554a53/third_party/blink/renderer/core/css/style_sheet.idl
   https://chromium.googlesource.com/chromium/src/+blame/3c5d7826be996563c73451a09bcbbddee8b3fed6/third_party/WebKit/WebCore/css/StyleSheet.idl
3. Firefox bug tracker:
   https://bugzil.la/846972
4. Firefox repository:
   https://github.com/mozilla/gecko-dev/blob/2169694adb1b691bcea73840bf3233de7379bcc5/dom/webidl/StyleSheet.webidl

* Sort browsers/features; Mark Firefoxes as true

mdn/browser-compat-data,chrisdavidmills/browser-compat-data,teoli2003/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data