Fix several bugs with Live Preview and LESS/SCSS files

* #8877: Fix StaticServerDomain to properly fall back to serving from disk
when Brackets-side code takes a pass on filtering a file that was listed in
`_rewritePaths`
* #8917: Wait until RemoteAgent is ready before first `onActiveEditorChange()`
call, which may immediately create a CSSPreprocessorDocument & immediately
try to do a HighlightAgent remote call.
* #8918: Implement `CSSPreprocessorDocument.isLiveEditingEnabled()`, which
LiveDevelopment assumes exists on all "live documents."
This is just to clarify that, afaik, there's no return value of next() and in general nothing we need to synchronously return from rewrite().  (CC @jasonsanjose in case I'm wrong about that!)