Support raw string interpolation in JSON

[kubukoz]

For example, this:

val bucket: Output[String] = ???
json"""{
  "key": "$bucket/*"
}"""

Fails to compile:

Failed to parse JSON (default values inserted at compile time):
  Unexpected character '"' at input index 17 (line 2, position 16), expected '}':
      "key": """/*"
               ^

ccc @lbialy