repl: fix terminal default setting and improve color check

[BridgeAR]

This makes sure that the described default behavior for the
terminal option is actually always used and not only when running
the REPL as standalone program.

The color detection is also improved by checking for hasColors()
if possible.

I also moved a few lines so that the code is logically combined instead of being spread out in the big REPL constructor.

Checklist

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