Typed Promise

[MOZGIII]

Motivation

js_sys::Promise -> js_sys::Promise<T = JsValue>

Better type system has obvious benefits.

Proposed Solution

1. Add a generic to the js_sys::Promise<T = JsValue>
2. Adjust the calls like then to take a Promise<T> and callback from T and return a differently-typed Promise<U>.
3. Adjust TypeScript codegen to allow specifying the T type (currently it is Promise<any>).

Alternatives

js_sys::Promise -> js_sys::Promise<T = JsValue, E = JsValue> - where E is the typed rejection; this is not how TypeScript types them though, and there are issues in general with errors typing due to the JavaScript language model itself.

Additional Context

I want to replace impl Future<Output = T> with js_sys::Promise<T> in my bindings where applicable.