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.
diff --git a/src/testdir/test_vim9_disassemble.vim b/src/testdir/test_vim9_disassemble.vim
index 557b90b..2862289 100644
--- a/src/testdir/test_vim9_disassemble.vim
+++ b/src/testdir/test_vim9_disassemble.vim
@@ -1190,7 +1190,7 @@
     def FuncTwo(): string
       return 'two'
     enddef
-    let g:res_FuncOne: string = execute('disass FuncOne')
+    g:res_FuncOne = execute('disass FuncOne')
   END
   writefile(lines, 'Xdisassemble')
   source Xdisassemble