Add label when completing a nested index (#119)

When doing auto-complete, it currently shows `*ast.Index` when it's nested Indexing because we don't follow up to the final ref
Instead of `ast.Index`, we can say `object attribute` which makes a bit more sense for users

pkg/server/completion.go

@@ -205,6 +205,8 @@ func typeToString(t ast.Node) string {
 		return "string"
 	case *ast.Import, *ast.ImportStr:
 		return "import"
+	case *ast.Index:
+		return "object field"
 	}
 	return reflect.TypeOf(t).String()
 }