respect magic trailing commas in return types

[jakkdl]

Description

Fixes #3018

I started out with modifying left_hand_split, doing a comprehensive rewrite of it to fully handle all manner of possible cases (see the length of the test file). But after reading more of the existing source code I started realizing that I was duplicating functionality, and in fact the only real thing that needed fixing was making sure that right_hand_split was applied to the function definition first when there's a trailing comma.
I also found a bug in the string splitting of return type and attempted to fix that at the same time ... but I ended up concluding that it's probably better solved in a separate PR (if at all, who even writes their return types like that??) with a very different approach.
I think there's a couple cases that can get improved, mainly cases like:

def aaaaaaaaaaaaaaaaa(
    bbbbbbbbbbbbbbbbbb,
) -> list[Ccccccccccccccccccccccccccccccccccccccccccccccccccc, Dddddd]: ...

would look much better if split on the right hand first. Detecting it also isn't terribly complicated ("split on rhs if the return type is longer than the param list"), I had it working in my original solution that rewrote left_hand_split.

Ended up doing some code duplication due to bracket_split_build_line having the side effect of normalizing the indent, which led to foo def() ->int: .... There's maybe better solutions doing stuff like adding a dont_normalize parameter to it.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation? (Probably not needed)

[github-actions]

diff-shades results comparing this PR (be48f7b) to main (9e9fdce). The full diff is available in the logs under the "Generate HTML diff report" step.

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│ 1 projects & 4 files changed / 16 changes [+4/-12]     │
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