feat: Rainbow parentheses

Rainbow parentheses come from Emacs, used to make languages like Lisp (S-Expressions) more easily readable. Today many code editors including VSCode and Kate optionally support them, which many turn on due to genuine readability improvement on deeply nested structures.

Describe the solution you'd like A new parameter in themes, allowing one to render the snippet with parentheses colored by palette[depth mod (len palette)].