Backslash ('\') not escaped in interpolated strings

[stkb]

Hi again,

In v3.6.3 (& 3.4.10 REPL)

// ok: "\\"
let a = @"\"

// not ok: `\`; should be `\\`
let b = $"\\"

// not ok: `\`; should be `\\`
let c = $@"\"

// not ok: `\`; should be `\\`
let d = @$"\"