CI check that enforces the formatting

[keunes]

Short description: Add a CI check that enforces the formatting, especially indent (number of spaces)

Location: site-wide

Why have this: It helps make/keep the code-base readable.

More info: I think we might want some includes to start at an indent bigger than 0, so that they integrate nicely in the output code. We may want to find a tool that's capable of handling this.

We'll also want to check only files touched by the commits/PR.

[keunes]

@ByteHamster @Rotzbua Maybe this could help us? https://github.com/cebe/indent

Or https://github.com/marketplace/actions/prettier-code-formatting / https://prettier.io/

Edit: since Prettier takes EditorConfig introduced in #244, that might be a better solution.