[Feature Request]: Custom IsolatedDeclarationPlugin output dir

What problem does this feature solve?

Problem

The default generation path of .d.ts in IsolatedDeclarationPlugin is a bit strange.

I don't understand why using Path::new(args.stable_id) and turn path into a form similar to /output + /module_path + /name.d.ts.

But what I expect is the form of /output + /name.d.ts, which means it will appear directly in the BundlerOptions.dir.

Maybe a custom path configuration would be better here.

What does the proposed API look like?

pub struct IsolatedDeclarationPlugin {
    pub strip_internal: bool,
    pub dir: Option<String>
}