Insert a XEP-0203 child into a serialized stanza (DOM-free string
surgery). Used when re-sending salvaged stanzas after a failed resumption,
so the receiving client can show the original send time.
Parameters
serialized: string
The serialized stanza.
name: string
The stanza's local tag name.
queuedAt: number
The original queueing time (ms since epoch).
Returns string
The stanza with a child, or the unmodified input if it
already carries one (or is malformed).
Insert a XEP-0203