Add rna-transcription exercise

[ErikSchierboom]

No description provided.

[BNAndras]

I think I'll ask on the Arturo Discord to confirm best practices here, but I suspect this should be toRna. That makes me think about using the to word to convert a DNA value to a RNA value like result: to :rna "A". For that, students will need to define a new RNA type. We can do that in an instructions append or add a custom DNA type at the top of the test suite so students hopefully get the hint about what's expected.

[ErikSchierboom]

@BNAndras We also need to consider the difficulty of the exercise. This is traditionally a relatively straightforward exercise, so the custom type with to: could maybe be postponed to a later exercise too. Something to consider.