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/json.pkl

@@ -18,6 +18,16 @@
 @ModuleInfo { minPklVersion = "0.31.0" }
 module pkl.json
 
+/// Annotate properties of classes and modules with this class to override how a [JsonRenderer]
+/// 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 [JsonRenderer].
+  name: String
+
+  render = (prop, renderer) -> if (renderer is JsonRenderer) Pair(name, prop.value) else prop
+}
+
 /// A JSON parser.
 ///
 /// JSON values are mapped to Pkl values as follows: