Separate function parameter from its attributes by a newline

Context

Hi, I'm the author of the crate bon. It exposes a proc macro that generates a builder for a function. Every function parameter can be configured with additional #[builder(...)] options.

Problem

For example, this function declares 4 parameters, two of which are optional (they have default values). As for me, the way rustfmt formatted this code doesn't look nice:

#[bon::builder]
fn example(
    #[builder(default = 1)] foo: u32,
    bar: u32,
    #[builder(default = 3)] baz: u32,
    fizz: u32,
) {
}

The attribute on the function's parameter was formatted on the same line with the parameter itself, even though the signature of the function already takes up multiple lines. It is harder to read the signature this way because names of function parameters aren't aligned

I'd expect the following formatting for this example of code instead:

#[bon::builder]
fn example(
    #[builder(default = 1)]
    foo: u32,
    bar: u32,
    #[builder(default = 3)]
    baz: u32,
    fizz: u32,
) {
}

Reproduce the issue using the Rust function examples in the report, then trace rustfmt's parameter and attribute formatting entry points. Done means multiline signatures place each parameter attribute on its own line before the parameter, matching the expected output.