"""Convert ISO 8601 timestamp string to milliseconds since epoch.

This function parses an ISO 8601 formatted timestamp string and converts it to the number of milliseconds since the Unix epoch (January 1, 1970, 00:00:00 UTC). The function automatically handles Zulu/UTC timezone indicators (Z).

Args:
    ts (str): ISO 8601 timestamp string, which may include a Zulu/UTC timezone indicator (Z).

Returns:
    int: The number of milliseconds since the Unix epoch as an integer.
"""
You should write code starting with:
from dateutil import parser

def iso_to_millis(ts: str) -> int:
    # TODO: Implement this function
    pass

