src: make process.env work with symbols for 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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

process