code_highlight() supports long strings and symbols

[moodymudskipper]

Closes #726

I used a better approach than what I used at the time for prettycode. This is less hacky and solves corner cases that I hadn't looked into back then.

It first looked like utils::getParseText() could solve the issue but unfortunately it doesn't parse long symbols (used also for names) so this is not enough. Hence I created get_parse_data() wrapping getParseDate() and borrowing elements from utils::getParseText().

[moodymudskipper]

@gaborcsardi Is there anything I can do to help moving this forward?

[gaborcsardi]

Thanks!