Add new lint fn_arg_mut_rebindings

[faeroon]

implements lint for checking argument rebindings as mut variable

changelog: [fn_arg_mut_rebindings]

• Followed lint naming conventions
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

Fixes #1657. Based on comments for previous PR #6245. Changes comparing to previous:

1. renames lint to satisfy naming convention
2. applies lint only to let mut x = x; statements with scope of function body
3. skips lint for trait implementation functions

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[github-actions]

Lintcheck changes for ca552cf

Lint	Added	Removed	Changed
clippy::fn_arg_mut_rebindings	5	0	0

This comment will be updated if you push new changes

[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.