add verbatim strings (@"...")

[Alexandre Moreno]

verbatim strings eschew all of the normal string escaping.

std.assertEqual(@"c:\negative""", 'c:\\negative"')

Verbatim strings start with an @ and can be enclosed in both single and double-quotes. More details on the spec: https://jsonnet.org/ref/spec.html

Depends on #7

[Alexandre Moreno]

solved in 5ed7aca