feat: syntax for verbatim string, and special char

[luochen1990]

1. add syntax support for verbatim string, which is a new syntax feature from idris 1.2.0
2. add syntax support for special chars. I checked the source code of idris for the syntax definition about string literal, and it point to here, and there is some document like that: The literal character is parsed according to the grammar rules defined in the Haskell report., so I just used the impl of Haskell and keep the name hsSpecialChar.