Experimental support for primitves bindings

[mlasson]

Description

This PR adds support for a new attribute [@js.runtime "string"] that allows bindings a js_of_ocaml pritimitive using Jsoo_runtime.Sys.external_ from ocsigen/js_of_ocaml#2086

For instance:

  val path: Ojs.t [@@js.runtime "node_path"]

expands to

  let (path : Ojs.t) = Jsoo_runtime.Sys.external_ "node_path"

I also added an extension of the existing "js.scope" attribute by adding an "@" to the parameter you can refer to a runtime primitive.

Example:

[@@@js.scope "@node_fs.promises"]
...
val unlink: string -> unit Promise.t [@@js.global]

expands to

let (unlink : string -> unit Promise.t) =
 fun (x50 : string) ->
   Promise.t_of_js Ojs.unit_of_js
     (Ojs.call
        (Ojs.get_prop_ascii (Jsoo_runtime.Sys.external_ "node_fs")
           "promises") "unlink" [|(Ojs.string_to_js x50)|])