Implement exercise: darts

[ee7]

Problem specifications

Exercise description
Canonical data

General notes

I'll set the other PRs to the new "draft" mode until I have time to comment on any implementation details in the PR description.

For simplicity, in config.json I added all new exercises after react, and set unlocked_by to null. So for now, I suggest merging the new exercises into config.json below react and around secret-handshake, in alphabetical order.

Alphabetical order could also help reduce the diff noise later if this suggested config.json ordering becomes the default.

After adding this wave of exercises we could consider whether any of the new exercises should be core, and change the track structure accordingly.

Thanks.

[amscotti]

Thanks for the new exercise!