SPICE-0024: Annotation converters (#1333)

This enables defining declarative key and/or value transformations in
cases where neither `Class`- nor path-based converters can be applied
gracefully. It is also the only way to express transforming the
resulting property names in `Typed` objects without applying a converter
to the entire containing type, which is cumbersome at best.

SPICE: https://github.com/apple/pkl-evolution/pull/26

stdlib/jsonnet.pkl

@@ -68,6 +68,16 @@ function ExtVar(_name: String): ExtVar = new { name = _name }
 ///   }
 /// }
 /// ```
+///
+/// The [Property] annotation can be used to change how a property name renders into Jsonnet.
+///
+/// Example:
+/// ```
+/// import "pkl:jsonnet"
+///
+/// @jsonnet.Property { name = "wing_span" }
+/// wingSpan: Int
+/// ```
 class Renderer extends ValueRenderer {
   extension = "jsonnet"
 
@@ -91,6 +101,16 @@ class Renderer extends ValueRenderer {
   external function renderValue(value: Any): String
 }
 
+/// Annotate properties of classes and modules with this class to override how a [Renderer]
+/// interprets a property's name.
+@Since { version = "0.31.0" }
+class Property extends ConvertProperty {
+  /// The new name to use for the annotated property when rendered by [Renderer].
+  name: String
+
+  render = (prop, renderer) -> if (renderer is Renderer) Pair(name, prop.value) else prop
+}
+
 /// An `importstr` construct that, when evaluated by Jsonnet, returns the content of a UTF-8 text file.
 ///
 /// To construct an [ImportStr], use method [ImportStr()].