Implement a CI step to detect unused dependencies 📦

[JamieSlome]

To ensure we don't unnecessarily hit "dependency bloat" nor merge pull requests with dependencies that are unused, we should introduce a GitHub Action or CI step to detect which dependencies are unused. This is a great first issue for someone looking to make their first contribution to the project 🎉

### Tasks
- [x] Detect unused dependencies and fail CI if more than 0
- [x] Detect unused `dependencies` and `devDependencies`
- [x] Only allow PR merges if the Action or CI step returns safely

I have previously come across depcheck - a nice lightweight NPM library that discovers unused dependencies.

[JamieSlome]

@marksmith388 - would you be interested in picking this one up? 🤔

[JamieSlome]

@tt-gideonaryeetey and @divinetettey - asked discussed, are you interested in picking this issue up? Let me know below and then I can assign to you if you're happy to 👍

[divinetettey]

Hi @JamieSlome yes I would pick this issue

[JamieSlome]

Achieved in #592. You're a rockstar @divinetettey 🚀