Trailing commas on Kotlin sources [has many
advantages](https://pinterest.github.io/ktlint/0.49.0/rules/standard/#trailing-comma-on-call-site):
- It makes version-control diffs cleaner – as all the focus is on the
changed value.
- It makes it easy to add and reorder elements – there is no need to add
or delete the comma if you manipulate elements.
- It simplifies code generation, for example, for object initializers.
The last element can also have a comma.
This PR doesn't go as far as require it, but tweaks KtLint to at least
allow it.
The two `.kt` files prove that the KtLint rules have been properly
disabled.
40d771d60f