patch 8.2.3656: Vim9: no error for an evironment variable by itself

Problem:    Vim9: no error for an evironment variable by itself.
Solution:   Give a "without effect" error. (closes #9166)

src/ex_eval.c

@@ -888,7 +888,8 @@ report_discard_pending(int pending, void *value)
 }
 
 /*
- * Return TRUE if "arg" is only a variable, register or option name.
+ * Return TRUE if "arg" is only a variable, register, environment variable or
+ * option name.
  */
     int
 cmd_is_name_only(char_u *arg)
@@ -911,6 +912,8 @@ cmd_is_name_only(char_u *arg)
 	    if (STRNCMP("l:", p, 2) == 0 || STRNCMP("g:", p, 2) == 0)
 		p += 2;
 	}
+	else if (*p == '$')
+	    ++p;
 	get_name_len(&p, &alias, FALSE, FALSE);
     }
     name_only = ends_excmd2(arg, skipwhite(p));

src/testdir/test_vim9_cmd.vim

@@ -584,6 +584,13 @@ def Test_use_register()
   END
   CheckDefAndScriptFailure(lines, 'E1207:', 2)
   &g:showbreak = ''
+
+  lines =<< trim END
+      $SomeEnv = 'value'
+      $SomeEnv
+  END
+  CheckDefAndScriptFailure(lines, 'E1207:', 2)
+  $SomeEnv = ''
 enddef
 
 def Test_environment_use_linebreak()

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3656,
 /**/
     3655,
 /**/