asyncjs: add then, catch for promise pipelining (#16871)

* asyncjs: add then
* improve tests, changelog, API
* fix cryptic windows error: The parameter is incorrect
* address comments

tests/js/tasyncjs_fail.nim

@@ -0,0 +1,22 @@
+discard """
+  exitCode: 1
+ outputsub: "Error: unhandled exception: foobar: 13"
+"""
+
+# note: this needs `--unhandled-rejections=strict`, see D20210217T215950
+
+import std/asyncjs
+from std/sugar import `=>`
+
+proc fn(n: int): Future[int] {.async.} =
+  if n >= 7: raise newException(ValueError, "foobar: " & $n)
+  else: result = n
+
+proc main() {.async.} =
+  let x1 = await fn(6)
+  doAssert x1 == 6
+  await fn(7).catch((a: Error) => (discard))
+  let x3 = await fn(13)
+  doAssert false # shouldn't go here, should fail before
+
+discard main()