Format typst files and require correct formatting of PRs

[rmburg]

See this test PR on my fork to see how the CI check would look. I'm open to change how this works, let me know what you think.

Maybe I should also document the use of Typstyle somewhere.

[andreasKroepelin]

There is a problem with formatting the book examples.

1. The code snippets are included in the book based on a line interval. These line numbers in the .md files would have to be adapted.
2. Often, the code is not indented on purpose because it would look strange in the book without the outer code.

[rmburg]

Apologies for the delay. I've updated the PR with your suggestions.
The leading whitespace issue in mdbook seems to be known: rust-lang/mdBook#1430
For now, I've excluded book/ from the checks.
Once the issue is fixed, we can update the checks to include the book as well.