Colour mode

[nojaf]

The good idea of the week award goes to @TheAngryByrd:
What is we had a tab where there is some colour coding between the AST nodes and the original syntax.
Highlighting SynExpr, SynPat, SynType, ...etc in a different code to illustrate the relation better to newcomers.

There would be value in getting newcomers better familiar with the SyntaxTree.