Release v1.3.0

feat: create dedicated kernel folder on `popcorn setup` (#46)

* feat: create dedicated kernel folder on `popcorn setup`

Instead of writing files directly into the current directory (which
overwrites existing files), `popcorn setup` now creates a subfolder
named after the problem directory (e.g. `softmax/`). If a folder with
that name already exists, a `-N` suffix is appended (`softmax-1/`,
`softmax-2/`, etc.) to avoid collisions.

* docs: update setup docs to reflect new project folder behavior

* style: fix rustfmt formatting in setup.rs