add some linting?

[Jabster28]

Is your feature request related to a problem? Please describe.
Lots of the PRs in this repo have a bunch of whitespace changes, because there aren't any linting rules. A lot of the code (e.g index.html and main.css) also has weird indentation that makes it hard to work with.

Describe the solution you'd like
Adding some code linters, such as ESLint and Prettier, maybe some pre-commit hooks or a PR github action too?

Describe alternatives you've considered
Maybe just a file or a bit in the readme that enforces a specific style, although that'd be much harder to maintain and enforce than having a package.json script to do it for you.

Additional context
Was planning to make a PR to fix the margin errors and the navbar clipping (but there's already a PR for that, haha) and noticed all of the weird indentation that's used and spare whitespace. Code would be a lot cleaner and PRs would be to review easier if we added an .eslintrc.json or something of the sorts.

airbnb has an npm package that has their own style guide, in case you don't want to spend too much time customising it

[Jabster28]

I'd also be happy to make a PR with their style guide, should you be interested.