patch 9.0.1982: vim9: clean up from v9.0.1955

Problem:  vim9: clean up from v9.0.1955
Solution: Fix a few remaining issues, improve error message

- Use `cl_exec`, the executing class, to check permissions in `get_lval()`.
- Handle lockvar of script variable from class.
- Add 'in class "Xxx"' to e_cannot_access_private_variable_str.

closes: #13222

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>

src/vim9.h

@@ -508,8 +508,9 @@ typedef struct {
 
 // arguments to ISN_LOCKUNLOCK
 typedef struct {
-    char_u	*string;	// for exec_command
-    int		is_arg;		// is lval_root a function arg
+    char_u	*lu_string;	// for exec_command
+    class_T	*lu_cl_exec;	// executing, null if not class/obj method
+    int		lu_is_arg;	// is lval_root a function arg
 } lockunlock_T;
 
 /*