Support for string literal type

[spaced]

See
https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#3.2.9
Example:

type CardinalDirection =
    "North"
    | "East"
    | "South"
    | "West";

Not sure how we should define it in scala because scala.Symbol is final
just a dumb idea:

object North = 'North
object South = 'South

interface JavaScriptFunction {
  def (a: North | South ): native
}

[sjrd]

Symbol has nothing to do with that. These are Strings, not Symbols.

There is nothing we can do but

1. Widening literal types to their general type (i.e., "North" becomes String), or,
2. Relying on SIP-23 literal types, which are not yet in Scala, and will only get there under -Xexperimental for a time