Major refactoring to use a Language Server

[mrmlnc]

Sorry, but your extension creates a large number of performance problems. Many of the issues in the VS Code repository refer to this extension. This is the most popular extension of its kind, so we need to make it better.

We need use Language Server and create background scanner for CSS files.

So, I'll start working on this soon (maybe this week). I create fork and then provide you PR.