Escaping newlines in single-line strings

[jiribenes]

Continuing the discussion from #410, do we actually want to be able to escape newlines like this:

val myString = "let's say I want to \
continue on the other line"      // ^ escapes a newline

// ~> myString = "let's say I want to continue on the other line"

Personally, I'm completely fine to forbid this, especially since we have multiple-line strings as of #410.