patch 9.0.1909: Vim9: problem calling class method from other class

Problem:  Vim9: problem calling class method from other class
Solution: Fix this problem, fix readonly object access, update error
          messages.

Calling a class method from another method without the class name prefix
doesn't work properly.

A readonly object variable is modifiable outside the class using a
nested object assignment.

Remove the unused E1338 error message.

Update error messages.

closes: #13116

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/vim9instr.c b/src/vim9instr.c
index ccec0bc..b9bbfd0 100644
--- a/src/vim9instr.c
+++ b/src/vim9instr.c
@@ -1899,9 +1899,15 @@
     // drop the argument types
     cctx->ctx_type_stack.ga_len -= argcount;
 
-    // For an object or class method call, drop the object/class type
+    // For an object or class method call, drop the object/class type.
     if (ufunc->uf_class != NULL)
-	cctx->ctx_type_stack.ga_len--;
+    {
+	// When a class method is called without the class name prefix, then
+	// the type will not be in the stack.
+	type_T *stype = get_type_on_stack(cctx, 0);
+	if (stype->tt_type == VAR_CLASS || stype->tt_type == VAR_OBJECT)
+	    cctx->ctx_type_stack.ga_len--;
+    }
 
     // add return type
     return push_type_stack(cctx, ufunc->uf_ret_type);