Add discord messaging timestamps

[warriorzz]

https://discord.com/developers/docs/reference#message-formatting

[BartArys]

What do we gain from introducing a DiscordTimestamp? Its sole reason for existence seems to be to convert it to a String. Why not just make it a String directly instead?

The entire lib uses kx's Instant for time already, it seems more prudent to introduce a toMessageFormat (or whatever you want to call it) extension that accepts your DiscordTimestampStyle to those types directly instead, and save the hassle of using an adapter type.

[warriorzz]

I didn't thought about that, but sounds good to me