Add Concept Exercise: `rpn-calculator`

[0xNeshi]

Towards #103

Scaffold https://github.com/exercism/cairo/blob/main/exercises/concept/rpn-calculator

TODO:

Use Rust's implementation as inspiration for code and metadata files https://github.com/exercism/rust/tree/main/exercises/concept/rpn-calculator

• [ ] implement instructions.md
• [ ] implement introduction.md
• [ ] implement hints.md
• [ ] implement ./meta/design.md
• [ ] implement the example solution in .meta/exemplar.cairo
• [ ] implement exercise stub at src/lib.cairo
• [ ] implement tests for the exercise at _tests/rpncalculator.cairo
• [ ] replace <your_gh_username> placeholder with your actual GH username in .meta/config.json
• [ ] add missing blurb to .meta/config.json