Refactor: distinguish variable responsibilities

[jayaddison]

This is largely an investigation aid for #1958. Separating the concerns of different variables may help discover the cause of the underlying problem(s).

NB: It's not ideal that the line.magic_trailing_comma variable name here shadows the mode.magic_trailing_comma variable name. They do have different types however.

[ambv]

Thanks! ✨ 🍰 ✨

[jayaddison]

No worries! I hope this is all fairly safe; there are definitely some small logical differences introduced here, and the scope for potential future impact from any change does worry me a bit. I'll watch out for any resulting problems, anyway.

[TBBle]

Per #1897 (comment), it looks like this PR missed updating /docs/reference/reference_functions.rst which broke the doc-gen job (which is annoyingly not run as part of CI for PRs).

[jayaddison]

D'oh, thanks @TBBle - I'll look into fixing that up.