feat(cli): `--format github` option

**What problem does this solve?**
GitHub Actions has rich rendering for errors / warning messages following [this format ](https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-an-error-message)

Example from the [griffe CLI](https://mkdocstrings.github.io/griffe/guide/users/checking/?h=output#github)

Adding this format to `harper-cli lint --format github` would allow a richer UX (e.g., for PR reviews) when using the CLI inside GitHub Actions

**Proposed Solution**
Adding this feature at the `harper-cli` level directly. Alternatively, users can create their own script to pipe the JSON into. Something like `harper-cli lint --format json | ./json_to_github.sh`

**Examples**
Show, don't tell.

**Component**
- [x] Core engine
- [] Plugin/Extension
- [ ] Other: _____

**Additional Context**
Any other relevant info.