cleanup: Reduce amount of legacy style
We've made some good progress in transitioning to the non-legacy style, so this seems like a good moment to pick up an old cleanup branch and bring it over the line:
- stop using string concatencation (just a couple of cases left)
- use new style for all arrays (again, not that many left)
- use new style for object literals (oh well, there are still a lot)
This does not only reduce the amount of legacy styling, but also the amount of legacy configuration, as rules that become unused can be dropped.