Merge: Improve highlight

highlight and nitlight need love.

The series fix some long issues. The main change is the improvement of the method `HightlightVisitor::enter_visit` that does only highlight the given node instead of the whole file.

Moreover, two new flags can be used to indicate how to deal with the context around the node given to highlight.

Note: A lot more love is needed, but this should really improve the current status of these modules.

Pull-Request: #1763
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>