Add new lint `filter_map_identity`
<!--
Thank you for making Clippy better!
We're collecting our changelog from pull request descriptions.
If your PR only includes internal changes, you can just write
`changelog: none`. Otherwise, please write a short comment
explaining your change.
If your PR fixes an issue, you can add "fixes #issue_number" into this
PR description. This way the issue will be automatically closed when
your PR is merged.
If you added a new lint, here's a checklist for things that will be
checked during review or continuous integration.
- \[x] Followed [lint naming conventions][lint_naming]
- \[x] Added passing UI tests (including committed `.stderr` file)
- \[x] `cargo test` passes locally
- \[x] Executed `cargo dev update_lints`
- \[x] Added lint documentation
- \[x] Run `cargo dev fmt`
[lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
Note that you can skip the above if you are just opening a WIP PR in
order to get feedback.
Delete this line and everything above before opening your PR.
-->
This commit adds a new lint named filter_map_identity.
This lint is the same as `flat_map_identity` except that it checks for the usage of `filter_map`.
---
Closes#6643
changelog: Added a new lint: `filter_map_identity`
ad9ceeebdc