Allow importing JS-specific Effekt libraries on jsWeb backend

[jiribenes]

Currently, it's not possible to import, say, mutable/map when working on the jsWeb backend.
This PR should fix that.

[jiribenes]

Tested quickly with

import mutable/map

def main() = {
  val m: Map[Int, String] = emptyMap()
  m.update(42, "hello")
  m.update(0, "zerooo")
  inspect(m.values)
  inspect(m.keys)
  inspect(m)
}

~>

$ effekt --backend=js-web minirepro.effekt

~>