Intent to ship: :read-only / :read-write pseudo-classes

*Summary: *Make our :-moz-read-only / :-moz-read-write pseudo-classes match the spec, and un-prefix them. *Bug: *Bug 312971 *Standard: *https://html.spec.whatwg.org/#selector-read-only // *Platform coverage: *All *

Re: Intent to ship: :read-only / :read-write pseudo-classes

A clarification given this came up in chat.mozilla.org: I'm keeping the support for parsing the old prefixed versions for backwards compat, aliased to the un-prefixed ones. We can potentially remove them, but probably at another time, specially given we're trying to minimize deprecations / bre