Add AbstractRange and StaticRange

This refactors large parts of the DOM Standard to reclassify Range
objects as "live ranges" rather than "ranges". The bits shared
between StaticRange and Range objects are put on a shared superclass
named AbstractRange.

This also introduces a "collapsed" definition and uses it throughout.

It does not contain everything from
https://w3c.github.io/staticrange/. I based this draft on the IDL
present in implementations, coupled with the desire for a superclass.
Extensions beyond this seem best addressed as follow-ups.

Tests: web-platform-tests/wpt#9967.

See also w3c/staticrange#1.