[WIP] Support lint level configuration in clippy.toml

[odysa]

changelog: Add support for lint level configuration in clippy.toml

Fixes #15128

Add support for configuring lint levels in clippy.toml using the same format as Cargo.toml. This allows users to move lint configurations from Cargo.toml to clippy.toml to keep dependency management and linting separate.

Example

clippy.toml:

[lints.clippy]
needless_return = "allow"
single_match = { level = "warn", priority = 5 }

[lints.rust]
dead_code = "allow"

[git-staus]

I wanted to mention that the following note in the README probably should be updated/removed as part of this PR as well:

rust-clippy/README.md

Lines 229 to 233 in 49e2f37

	> **Note**
	>
	> `clippy.toml` or `.clippy.toml` cannot be used to allow/deny lints.

Additionally, the "Clippy book" confusingly seems to contain very similar information to the README. Perhaps a paragraph on this (I guess also mentioning both a ~/.cargo/clippy.toml or a project-local .clippy.toml) should also be added to the "Allowing/Denying Lints" section there as well.

I am not sure to what standards changes to the documentation should conform; I just thought I'd point it out.

[rustbot]

☔ The latest upstream changes (possibly #17369) made this pull request unmergeable. Please resolve the merge conflicts.