vm: use missing validator

The `vm` lib module's `isContext()` function should use a validator.

PR-URL: https://github.com/nodejs/node/pull/38935


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>