patch 9.0.0481: in :def function all closures in loop get the same variables

Problem:    In a :def function all closures in a loop get the same variables.
Solution:   Use a separate list of variables for LOADOUTER and STOREOUTER.
            Not copied at end of loop yet.

src/structs.h

@@ -1656,7 +1656,7 @@ typedef enum {
 
 /*
  * Structure to hold info for a user function.
- * When adding a field check copy_func().
+ * When adding a field check copy_lambda_to_global_func().
  */
 typedef struct
 {
@@ -1741,7 +1741,8 @@ typedef struct
 #define FC_NOARGS   0x200	// no a: variables in lambda
 #define FC_VIM9	    0x400	// defined in vim9 script file
 #define FC_CFUNC    0x800	// defined as Lua C func
-#define FC_COPY	    0x1000	// copy of another function by copy_func()
+#define FC_COPY	    0x1000	// copy of another function by
+				// copy_lambda_to_global_func()
 #define FC_LAMBDA   0x2000	// one line "return {expr}"
 
 #define MAX_FUNC_ARGS	20	// maximum number of function arguments
@@ -2096,10 +2097,17 @@ struct funcstack_S
 
 typedef struct outer_S outer_T;
 struct outer_S {
-    garray_T	*out_stack;	    // stack from outer scope
+    garray_T	*out_stack;	    // stack from outer scope, or a copy
+				    // containing only arguments and local vars
     int		out_frame_idx;	    // index of stack frame in out_stack
     outer_T	*out_up;	    // outer scope of outer scope or NULL
     partial_T	*out_up_partial;    // partial owning out_up or NULL
+
+    garray_T	*out_loop_stack;    // stack from outer scope, or a copy
+				    // containing only vars inside the loop
+    short	out_loop_var_idx;   // first variable defined in a loop
+				    // in out_loop_stack
+    short	out_loop_var_count; // number of variables defined in a loop
 };
 
 struct partial_S