[Feature Request] Add Map and Set bindings in Js module

[searleser97]

Currently there is no way to use Map or Set with rescript natively,
alternatively we have to use some library like
rescript-js or write our own bindings,
I think they should be included by default in the Js module.

[cannorin]

Very nice to have these, even only the placeholder types (as in #5539).

[cristianoc]

Anyone up for a PR?

[cannorin]

I'll do the placeholder (as a first step) this week if no one else is going to do the full binding soon.

[cknitt]

Bindings for Map, Set, WeakMap, WeakMap are in the new rescript-core standard library.

I have created a PR there to use the placeholder types that you added: rescript-lang/rescript-core#143