Add StdErr to StdIoExtension

[Michael1993]

Closes #650

Proposed commit message:

Add StdErr to StdIoExtension (#650 / #653)

Adds a new parameter to the StdIoExtension, StdErr, which
replaces System.err the same way StdOut replaces
System.out.

Closes: #650
PR: #653

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[Michael1993]

I did a lazy thing with the documentation. 😬 Let me know if this is okay.

[Bukama]

Just a few notes, you may consider (or not :D ). Thanks for PR!

[Bukama]

Thank you :) Nice PR