Separate out "trivia" from "notation"

The usual definition of "trivia" are "things that can be removed from the input without affecting the semantics of the output of the parse" and includes whitespace, comments (and depending on who you ask, docstrings). However, when JuliaSyntax talks about "trivia" this includes everything that does not end up in the final AST and includes fixed keywords, notation, etc. We should separate these categories so that downstream users can make use of this information. In some circumstances, it might make sense to print code without trivia (e.g. when space constrained), or to change the highlighting of trivial, but keeping notation intact.