added asyncjs standard library module (#6841)

tests/js/tasync.nim

@@ -0,0 +1,26 @@
+discard """
+  disabled: true
+  output: '''
+0
+x
+'''
+"""
+
+import asyncjs
+
+# demonstrate forward definition
+# for js
+proc y(e: int): Future[string]
+
+proc x(e: int) {.async.} =
+  var s = await y(e)
+  echo s
+
+proc y(e: int): Future[string] {.async.} =
+  echo 0
+  return "x"
+
+
+
+discard x(2)
+