patch 8.2.4312: no error for using :vim9script in a :def function

Problem:    No error for using :vim9script in a :def function.
Solution:   Give an error when compiling.

src/testdir/test_vim9_script.vim

@@ -1227,6 +1227,7 @@ def Test_vim9script_fails()
   v9.CheckScriptFailure(['vim9script', 'const str = "asdf"', 'str = "xxx"'], 'E46:')
 
   assert_fails('vim9script', 'E1038:')
+  v9.CheckDefFailure(['vim9script'], 'E1038:')
 enddef
 
 def Test_script_var_shadows_function()

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4312,
 /**/
     4311,
 /**/

src/vim9compile.c

@@ -3138,6 +3138,10 @@ compile_def_function(
 			line = compile_script(line, &cctx);
 		    break;
 
+	    case CMD_vim9script:
+		    emsg(_(e_vim9script_can_only_be_used_in_script));
+		    goto erret;
+
 	    case CMD_global:
 		    if (check_global_and_subst(ea.cmd, p) == FAIL)
 			goto erret;