[PROTOTYPE] Live Preview source map support

Work in progress...

* Supports CSS selector highlighting by using source maps to find the selector from a SASS/LESS file line:col location

That's it. It doesn't support live edits to SASS/LESS since we need to save the file in-order for SASS/LESS to compile the output css. We can use source maps to discover which source files to compile I believe, but it might be unnecessary work since we can also use file watchers to watch the generated CSS file and just do a dumb reload for the entire page or try something smarter by updating the CSS in-place as we do currently.

I used this test project with source maps from bootstrap SASS and LESS implementations https://github.com/jasonsanjose/brackets-source-map-demo-files.

This PR doesn't actually get us very far with respect to features, at least not initially. Source maps don't account for things like variables and mix-ins, so the only visible feature is CSS selector highlighting.