feat: add PKL schema for message formats

pkg/format/init.pkl.go

@@ -0,0 +1,9 @@
+// Code generated from Pkl module `format`. DO NOT EDIT.
+package format
+
+import "github.com/apple/pkl-go/pkl"
+
+func init() {
+	pkl.RegisterMapping("format", Format{})
+	pkl.RegisterMapping("format#Macaroon", Macaroon{})
+}