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 CSSPreprocessorDocumetn & immediately
try to to a HighlightAgent remote call.
* #8918: Implement CSSPreprocessorDocument.isLiveEditingEnabled(), which
LiveDevelopment assumes exists on all "live documents."
Merge pull request #8920 from adobe/pflynn/livedev-less-fixes-release

Fix several bugs with Live Preview and LESS/SCSS files