Add initial support for raw string literals

[dkearns]

This is incomplete but probably does a decent job highlighting correct code.

TODO:

• Single-line and multi-line raw strings should be handled separately and more strictly.
• Invalid delimiter sequences should be highlighted as errors.
• Normal strings shouldn't obscure errors by matching in invalid raw string literal syntax.
• Should raw strings be coloured distinctly?

[nickspoons]

Thanks @dkearns. To answer your question, in my opinion no, raw strings shouldn't be highlighted differently to other strings. Despite the new semantics they are just strings at the end of the day, and people have expectations of those basic highlight colours. People who want them coloured differently can always link csRawString to something else in their personal config if they choose.