Add new concept mapsets and concept exercise gotta-snatch-em-all

[jiegillet]

This PR add new concept and concept exercise, both forked from the Elm track.

Full disclosure, I used AI (via Cursor) to help translate from one language to the other, and event though I combed through everything and edited a lot of things, it was a huge time saver.

There might be stylistic differences between tracks that I'm blind to, I'm happy to change anything.

The CI is expected to fail until the about.md is finalized, then I'll copy it to introduction.md (let me know if you think some parts should be cut down) and use the generator to add the intro file to the exercise.

[github-actions]

Thank you for contributing to exercism/elixir 💜 🎉. This is an automated PR comment 🤖 for the maintainers of this repository that helps with the PR review process. You can safely ignore it and wait for a maintainer to review your changes.

Based on the files changed in this PR, it would be good to pay attention to the following details when reviewing the PR:

• General steps

  • 🏆 Does this PR need to receive a label with a reputation modifier (x:size/{tiny,small,medium,large,massive})? (A medium reputation amount is awarded by default, see docs)

• Any exercise changed

  • 👤 Does the author of the PR need to be added as an author or contributor in <exercise>/.meta/config.json (see docs)?
  • 🔬 Do the analyzer and the analyzer comments exist for this exercise? Do they need to be changed?
  • 📜 Does the design file (<exercise>/.meta/design.md) need to be updated to document new implementation decisions?

• Concept exercise changed

  • 🌲 Do prerequisites and practices in config.json need to be updated?
  • 📖 Does the concept introduction provide all necessary information to solve this exercise?

• Concept exercise tests changed

  • ⚪️ Are all tests un-skipped?
  • 🔢 Are all tests annotated with @tag task_id?
  • 🐈 Can all tests be understood by reading the test's block only (e.g. no module attributes, no setup functions)?

• Concept changed

  • 👤 Does the author of the PR need to be added as an author or contributor in <concept>/.meta/config.json (see docs)?
  • 📖 Does the concept introduction provide all necessary information to solve this exercise?

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