patch 8.2.1329: Vim9: cannot define global function inside :def function

Problem:    Vim9: cannot define global function inside :def function.
Solution:   Assign to global variable instead of local. (closes #6584)

src/structs.h

@@ -1546,6 +1546,7 @@ typedef enum {
 
 /*
  * Structure to hold info for a user function.
+ * When adding a field check copy_func().
  */
 typedef struct
 {
@@ -1618,6 +1619,7 @@ 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 MAX_FUNC_ARGS	20	// maximum number of function arguments
 #define VAR_SHORT_LEN	20	// short variable name length