Default syntax-highlight #:python-executable on Windows

[Greg Hendershott]

Arising from #208.

syntax-highlight's #:python-executable defaults to "python", always. Instead, the default should vary and be "python.exe" when on Windows.

Gist of it would be something like (if (eq? (system-type) 'windows) "python.exe" "python").

Note: This would be an easy item for a first-time contributor, and/or someone using Frog on Windows.