src: make process.env work with symbols in/delete

The getter for process.env already allows symbols to be used, and `in`
operator as a read-only operator can do the same.

`delete a[b]` operator in ES always returns `true` without doing
anything when `b in a === false`. Allow symbols in the deleter
accordingly.

PR-URL: https://github.com/nodejs/node/pull/11709


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>