Add exercise: two-fer

[simisc]

Add the Two-Fer practice exercise.

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[ErikSchierboom]

@simisc Thanks! I've added @pclausen as a reviewer

[iHiD]

Just to weigh in:

• This passes the CI [1]
• The "quality" of the example file doesn't really matter as it's a practice exercise
• The tests look really clean

So let's give @pclausen a day or two to reply, but I'm in favour of taking a relatively optimistic merging approach to this if he doesn't get round to it! :)

Thanks @simisc!

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[1]

[ErikSchierboom]

Absolutely. But I did want to give @pclausen the chance to review :)

[simisc]

I've reduced indentation from 3 to 2 spaces for consistency with the other exercises in the track. The online Fortran editor also uses 2 spaces, so the stub file should start with the same formatting.

[ErikSchierboom]

Thanks!