patch 8.2.0725: Vim9: cannot call a function declared later in Vim9 script

Problem:    Vim9: cannot call a function declared later in Vim9 script.
Solution:   Make two passes through the script file.

src/vim.h

@@ -2133,6 +2133,7 @@ typedef enum {
 // Flags for assignment functions.
 #define LET_IS_CONST	1   // ":const"
 #define LET_NO_COMMAND	2   // "var = expr" without ":let" or ":const"
+#define LET_REDEFINE	4   // variable can be redefined later
 
 #include "ex_cmds.h"	    // Ex command defines
 #include "spell.h"	    // spell checking stuff