Migrate to ESLint flat config & neostandard

[MetRonnie]

Supersedes #1747

The flat config format in ESLint 9 in some ways makes more sense than the old format, but the documentation is rather lacking. Essentially, you've got an array of configuration objects, that ESLint combines.

I have moved from standard to neostandard, as standard is apparently not maintained anymore. Unfortunately, neostandard's documentation is also rather lacking. It aims to replace the need for a normal ESLint config, except that only seems to work for very simple use cases. But it can stil be combined with a normal flat ESLint config. Several of the ESLint plugins we use are bundled with neostandard, so I've removed them from our dependencies.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] No new tests are needed
• [x] No changelog or docs needed