Add parsing for Verbatim Strings

[kindlich]

Verbatim strings: @"xxx" is a verbatim string and can contain newlines and anything else you want, except ". To use ", double the quote. Exists both in single and double quote variants. No escape character processing. Multiple lines allowed.

Currently, while the Tokenizer does know of TokenTypes for verbatim strings, these are not used in the parser for actually parsing the strings, therefore they cannot be used right now.