Add grains exercise

[BNAndras]

No description provided.

[Stargator]

@ErikSchierboom, just checking are we accepting new exercises? Or is that a new track decision?

[ErikSchierboom]

@ErikSchierboom, just checking are we accepting new exercises? Or is that a new track decision?

I don't think so. The reason this PR wasn't auto-closed was that we have an expired token.

That said, the PR looks good. Are you happy to review or want me to do it?

[Stargator]

I'm fine with reviewing it.

[Stargator]

Sorry for the delay in this review.

Please update with the latest version of grains, they changed some of the wording. Already approved and I will merge it when the update is made.

[BNAndras]

 ./bin/configlet sync --metadata --docs --filepaths --tests include -uye grains
Updating cached 'problem-specifications' data...
Checking exercises...
The `grains` exercise has up-to-date docs, filepaths, metadata, and tests!

I don't see anything to update for grains. If I run configlet sync for all exercises, I see that acronym, allergies, armstrong-numbers, darts, isogram, and phone-number have unsynced docs.