feat: add duplicate_fn_bodies lint

[orielhaim]

Motivation

This adds a small first step towards detecting duplicated code in Clippy.

duplicate_fn_bodies looks for functions in the same crate whose bodies are textually identical, ignoring whitespace and comments.

For now I intentionally kept the lint very conservative. It only handles Type-1 style clones: renamed locals, changed literals, reordered statements, etc. are considered different bodies. I wanted to start with the case that can be detected with very high confidence before considering anything more structural.

Checklist

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

changelog: new lint: [duplicate_fn_bodies]

[rustbot]

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[github-actions]

Lintcheck changes for c389da1

Lint	Added	Removed	Changed
clippy::duplicate_fn_bodies	2066	0	0

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