C#: Add verbatim strings (like PowerShell's Here-String)

C# has multi-line "verbatim strings" of the form:

$var =  @"string content
and more content"

There's no single-quote version and regular escape chars (\) aren't recognized in the string --- only double double-quotes (""), which become a single double-quote.  This also checks that the '@' isn't escaped with a \.