patch 8.2.3021: spaces allowed between option name and "!", "?", etc.

Problem:    Spaces allowed between option name and "!", "?", etc.
Solution:   Disallow spaces in Vim9 script, it was not documented.
            (closes #8408)

src/option.c

@@ -1306,7 +1306,8 @@ do_set(
 	    // remember character after option name
 	    afterchar = arg[len];
 
-	    // skip white space, allow ":set ai  ?"
+	    if (!in_vim9script())
+		// skip white space, allow ":set ai  ?", ":set hlsearch  !"
 		while (VIM_ISWHITE(arg[len]))
 		    ++len;
 

src/testdir/test_vim9_script.vim

@@ -3937,6 +3937,26 @@ def Test_mapping_line_number()
   delfunc g:FuncA
 enddef
 
+def Test_option_modifier()
+  var lines =<< trim END
+      set hlsearch &  hlsearch  !
+      call assert_equal(1, &hlsearch)
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      set hlsearch &
+  END
+  CheckScriptFailure(lines, 'E518:')
+
+  lines =<< trim END
+      vim9script
+      set hlsearch &  hlsearch  !
+  END
+  CheckScriptFailure(lines, 'E518:')
+enddef
+
 " Keep this last, it messes up highlighting.
 def Test_substitute_cmd()
   new

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3021,
 /**/
     3020,
 /**/