[enh] Merge and sort comma-separated attribute parameters for known attributes

I would like to propose cargo fmt to merge and sort attribute params in cases where the attribute is well known and wouldn't cause any behavioral changes (?):

From:

#![allow(unused_variables, unused_imports, unused_macros)]
#![allow(clippy::eq_op)]
#![allow(clippy::format_in_format_args)]
#![allow(clippy::print_literal)]
#![allow(named_arguments_used_positionally)]

into

#![allow(
    clippy::eq_op,
    clippy::format_in_format_args,
    clippy::print_literal,
    named_arguments_used_positionally,
    unused_imports,
    unused_macros,
    unused_variables
)]

P.S. Should this be a fmt or a clippy action?

Use the examples in the issue as initial acceptance cases. First determine whether this belongs in rustfmt or clippy, then identify the existing attribute-formatting tests; done means known comma-separated parameters are merged and sorted without behavioral changes.