patch 8.2.2982: Vim9: future commands are not reserved yet

Problem:    Vim9: future commands are not reserved yet.
Solution:   Add commands to be implemented later.  Make "this" a reserved
            name.

src/vim9script.c

@@ -979,6 +979,7 @@ static char *reserved[] = {
     "true",
     "false",
     "null",
+    "this",
     NULL
 };