encoding/jsonschema: add several validators

- minLength
- maxLength
- additionalItems

Change-Id: I09d9e353914b0fece70a26d97db5c4665c4e5f8b
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5651
Reviewed-by: roger peppe <rogpeppe@gmail.com>
diff --git a/encoding/jsonschema/decode.go b/encoding/jsonschema/decode.go
index ac2a4af..72bd2cb 100644
--- a/encoding/jsonschema/decode.go
+++ b/encoding/jsonschema/decode.go
@@ -237,8 +237,7 @@
 		len(s.patterns) > 0 ||
 		s.title != "" ||
 		s.description != "" ||
-		s.obj != nil ||
-		s.list != nil
+		s.obj != nil
 }
 
 const allTypes = cue.NullKind | cue.BoolKind | cue.NumberKind | cue.IntKind |