Some more JS modernization

Another batch of cleanups to modernize the codebase:

• replace Array.indexOf() with Array.includes() where we aren't interested in the index
• replace C-style for loops with for..of, for..in and similar constructs

Other suggestions¹ are welcome.

1. other than moving to ES6 classes, which will break inheritance in extensions and thus requires some heads-up to extension authors ↩