Define the labels used in this repo.

This commit adds a `.github/labels.yml` file, which contains the full list of labels that this repo can use. This file is a combination of the `.appends/.github/labels.yml` file and the Exercism-wide labels defined in https://github.com/exercism/org-wide-files/blob/main/global-files/.github/labels.yml.