patch 8.2.1024: Vim9: no error for using "let g:var = val"

Problem:    Vim9: no error for using "let g:var = val".
Solution:   Add an error.

src/structs.h

@@ -67,6 +67,8 @@ typedef struct terminal_S	term_T;
 typedef struct VimMenu vimmenu_T;
 #endif
 
+// maximum value for sc_version
+#define SCRIPT_VERSION_MAX 4
 // value for sc_version in a Vim9 script file
 #define SCRIPT_VERSION_VIM9 999999