fix: improper conversion of iso8601 date to unix timestamp

[scinscinscin]

What?

Fixes the cog_iso8601_to_unix_ms function to properly account for milliseconds in ISO8601 dates

Why?

Previously, the function was broken because the double second field was being cast to a uint_64 before being multiplied by 1000, thus the decimal was being rounded For example, 54.043 was being cast to 54 before being multiplied by 1000, thus the result was 54000 instead of 54043

How?

The solution was to add parenthesis so that the multiplication happens before the casting.

Testing?

I tested my changes by creating another bot in discord.js and comparing the timestamps there with the timestamps generated by concord.

Screenshots (optional)

Here's what the behavior looked like before I made these changes (notice that Concord's timestamps are rounded to the nearest thousands):

Here's what the behavior is like after I made the changes: