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

pkl-core/src/test/files/LanguageSnippetTests/input-helper/api/annotationConverter.pkl

@@ -0,0 +1,19 @@
+class LineComment extends ConvertProperty {
+  text: String
+
+  valuePrefix: String = ""
+  prefix: String
+  suffix: String = ""
+
+  render = (prop, renderer) ->
+    Pair(prop.key, new RenderDirective {
+      text =
+        List(
+          valuePrefix,
+          (renderer as ValueRenderer).renderValue(prop.value),
+          prefix,
+          outer.text,
+          suffix,
+        ).join("")
+    })
+}