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/structs.h

@@ -4595,6 +4595,16 @@ typedef struct lval_S
 				// ignore the rest.
 } lval_T;
 
+/**
+ * This may be used to specify the base type that get_lval() uses when
+ * following a chain, for example a[idx1][idx2].
+ */
+typedef struct lval_root_S {
+    typval_T	*lr_tv;
+    class_T	*lr_cl_exec;	// executing class for access checking
+    int		lr_is_arg;
+} lval_root_T;
+
 // Structure used to save the current state.  Used when executing Normal mode
 // commands while in any other mode.
 typedef struct {