fix newline width calc in combine w/ comments

[calebcartwright]

Resolves #4031, refs #4110

This address the binding formatting in #4110, but doesn't include any updates for the sub pattern formatting

[calebcartwright]

I assume this is indeed a fix and that we don't want to use the full length of next_str. Otherwise next_str can start to exceed the shape width pretty quickly.

For example:

fn foo() {
    match glyph {
        found @ Glyph {
            data: Gl::Com { .. },
            foo: bar,
        } => (),
        _ => (),
    };
}

Remains unchanged by rustfmt, but adding a few more items to the struct (or longer names) will increase the total next_str length and fail the conditional check resulting in a newline insertion.

fn foo() {
    match glyph {
        found @ Glyph {
            data: Gl::Com { .. },
            foo: bar,
            baz: qux,
        } => (),
        _ => (),
    };
}

would be reformatted to the below without this change

fn foo() {
    match glyph {
        found
        @
        Glyph {
            data: Gl::Com { .. },
            foo: bar,
            baz: qux,
        } => (),
        _ => (),
    };
}

[topecongiro]

LGTM. Thank you!