Add Racket lexer, and switch *.rkt to it from Scheme.

- .rkt, .rktd, .rktl files
- Accept   \u03BB lambda character
- Long list of keywords and very long list of builtins
- Numeric constants like -inf.0, +inf.0, -min.0, +max.0 and friends
- Add test (and adjust Scheme test)
- Add to visual/samples