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."