Add Concept Exercise: gross-store

[0xNeshi]

Towards #103

Scaffold https://github.com/exercism/cairo/blob/main/exercises/concept/gross-store

TODO:

• use Go's implemented concept exercise for inspiration, e.g. https://github.com/exercism/go/tree/main/exercises/concept/gross-store
  • implement instructions.md
  • implement introduction.md
  • implement design.md
  • implement hints.md
• implement the example solution in .meta/exemplar.cairo
• implement the exercise stub at src/lib.cairo
• implement tests for the exercise at tests/gross_store.cairo
• replace <your_gh_username> placeholder with your actual GH username in .meta/config.json