Add rule that last line in multi-line collection should have comma

Currently there's no rule that expresses our stance on this subject. Additionally, judging by the discussions we've had so far in chat, there seem to be differing opinions on the subject.

Subsequently, there are two things that should be resolved in this PR:

First of all, as a question to each person: if the options are **comma** at the end of the last line or **no comma**, what do you prefer?

The second thing is to decide (based on the answers) whether we should have a rule at all.

My point of view is that there should be a rule and that there *should* be a comma at the end.