refactor(module): simplify and fix configMerge

[KristjanESPERANTO]

I had a hard time understanding configMerge - the old arguments slicing and nested loops took me a while. The jsdoc didn't help much. After it finally clicked, I rewrote it to be readable: rest param + a simple for...of instead of the while-stack.

And there's one real behavior fix. With this default and user config:

defaults: { foo: { a: 1 } }
config:   { foo: [10, 20] }

the old code merged them into { foo: { 0: 10, 1: 20, a: 1 } }. Now the array just replaces the object -> { foo: [10, 20] }.
I added a small test for that case.

Only affects modules using configDeepMerge: true that override an object default with an array, so the impact is tiny.