chore: Extract buildin* to FunctionBuilder

[He-Pin]

Motivation:

Make the buildin* helper function accessible without extending the Std() class.

Then I can just extend the FunctionBuilder, build the additionalNativeFunctions, and then pass it to the Std()'s constructor.

otherwise:

object JsonnetStdExt extends Std(Map.empty) {
  val extFunctions: Map[String, Val.Func] = Map(
    builtin("objectReplaceKey", "obj", "key", "newKey") {
      (pos, ev, o: Val.Obj, key: String, newKey: String) =>
        val bindings: Array[(String, Val.Obj.Member)] = for {
          k <- o.visibleKeyNames
          v = o.value(k, pos.fileScope.noOffsetPos)(ev)
        } yield {
          val newKeyName = if (k == key) newKey else k
          (newKeyName, new Val.Obj.ConstMember(false, Visibility.Normal, v))
        }
        Val.Obj.mk(pos, bindings)
    }
  )
}

[He-Pin]

@stephenamar-db I have rebased on the current master.