encoding/jsonschema: expose API of JSON Schema decoder

Change-Id: I191d7bc2394486fa63050fca6721f65aa0589b22
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4760
Reviewed-by: Antoine Pelisse <apelisse@google.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/jsonschema/decode_test.go b/encoding/jsonschema/decode_test.go
index da64a24..dddb127 100644
--- a/encoding/jsonschema/decode_test.go
+++ b/encoding/jsonschema/decode_test.go
@@ -75,8 +75,7 @@
 				t.Fatal(err)
 			}
 
-			// TODO: use actual API.
-			expr, err := decode(fullpath, in)
+			expr, err := Extract(in, &Config{ID: fullpath})
 			if err != nil && errout == nil {
 				t.Fatal(errors.Details(err, nil))
 			}