builtin, cgen: support FieldData.unaliased_typ, use it for generalising x.json2 (able to encode type aliased struct fields) (#16469)

vlib/builtin/builtin.v

@@ -110,12 +110,13 @@ pub:
 // FieldData holds information about a field. Fields reside on structs.
 pub struct FieldData {
 pub:
-	name      string
-	attrs     []string
-	is_pub    bool
-	is_mut    bool
-	is_shared bool
-	typ       int
+	name          string
+	attrs         []string
+	is_pub        bool
+	is_mut        bool
+	is_shared     bool
+	typ           int
+	unaliased_typ int
 }
 
 pub enum AttributeKind {