Add split_filter_map_ok lint

[vitkyrka]

Add a new lint identical to lines_filter_map_ok except that it checks for std::io::Split instead of std::io::Lines.

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changelog: [split_filter_map_ok]: add lint

[rustbot]

r? @samueltardieu

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The reviewer was selected based on:

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

☔ The latest upstream changes (possibly #17552) made this pull request unmergeable. Please resolve the merge conflicts.