Support previews of interpolated JSON values

[kubukoz]

For example, given the following interpolated JSON:

val bucket: Output[String] = ???

Something("thing", SomethingArgs(policyJson = json"""{
  "key": "$bucket/*"
}"""))

it should be possible to preview that policyJson is going to look like {"key": "$PLACEHOLDER/*"}. My understanding is that the entire JSON value will be a placeholder because it desugars to the equivalent of bucket.map { ... } and is too opaque to be inspected.

cc @lbialy