Unsafe host-specific bindings

[cometkim]

We have this in the Core module.

@val external window: Dom.window = "window"
@val external document: Dom.document = "document"
@val external globalThis: {..} = "globalThis"

(ref: https://github.com/rescript-association/rescript-core/blob/22642ea/src/RescriptCore.res#L47-L49)

However, using window and document is not safe because

• They are only available on the HTML DOM. @cknitt suggest to use option for it. Add Js.globalThis object binding rescript-compiler#6909 (comment)
• Their scope isn't specified explicitly to globalThis.

I also think that the compiler and the Core should not promote specific hosts. window & document are part of the HTML and has nothing to do with Node.js. global is Node.js specific and has nothing to do with browsers.

IMO Core should delegate bindings to third-parties (e.g. rescript-webapi) and keep the representation in JavaScript itself.