introduce stylistic + update devDependencies

[KristjanESPERANTO]

Add @stylistic/eslint-plugin with single quotes enforcement. Foundation for future style rules - more can be enabled gradually.

The long-term goal would be to follow all the recommended stylistic rules.

[HolgerJeromin]

"the world" of javascript is moving away from stylistic eslint rules to https://prettier.io

[KristjanESPERANTO]

Prettier would cause massive unreviewable changes here, so I took an incremental approach. But I'm flexible - I've already set up a prettier branch to compare the impact if we wanted to switch.

Technically, linter and formatter are different roles, but I think it's good if both are handled by one tool. Some parts of the ecosystem seems to be moving that way (Biome, ESLint's expansion since v9). Happy to go either way.

[KristjanESPERANTO]

To make some progress here, I'm merging this. I'm still open to other solutions, but for now I've decided to go with "stylistic" because

1. it doesn't result in a massive amount of code changes in the first step and
2. aligns with https://github.com/opening-hours/opening_hours.js/.