Add os.environ.setdefault(str, str) functionality

[avitkauskas]

Implements os.environ.setdefault(key, default = "") functionality as in python (closes #583).

Set the value of the key environment variable to default if it did not exist and return it. Return the value from the environment if it was already set.

import os

a = os.environ.setdefault("A", "default_value")

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