Offer a Github CI action for harper-cli

**What problem does this solve?**

Easy integration of Harper into the CI of projects hosted on Github.

**Component**
- [ ] Core engine
- [ ] Plugin/Extension
- [x] Other: Github CI Actions

**Additional Context**

We're looking at integrating Harper in the bevy-website repo during Rustweek. It would be really cool if the `harper-cli` tool was easier to integrate, even if it's still in an experimental stage :)

Start by locating the harper-cli entry point and reviewing the requirements for a GitHub CI Action that can invoke it. The work is complete when Harper can be integrated into GitHub-hosted projects such as bevy-website through a documented, working action.

Written by the indexing model from the issue text.