Improve implementation of LS.getclass and LS.classname, make it more consistent.

luprex/cpp/core/util.cpp

@@ -188,6 +188,11 @@ bool is_lua_id(string_view str) {
     return true;
 }
 
+bool is_lua_classname(string_view s) {
+    if (s == "_G") return false;
+    return is_lua_id(s);
+}
+
 bool is_lua_comment(string_view s) {
     int start = 0;
     while ((start < int(s.size())) && ((s[start]==' ') || (s[start]=='\t'))) start++;