Add first join message type for Discord

[diademiemi]

Information

This PR partially closes #5155

Details

Proposed feature:
Add a new message type for when a player has joined for the first time.

This adds a new type first-join with the default placeholder ":arrow_right: {displayname} has joined the server for the first time!".

Environments tested:

OS: Fedora 36

Java version: 17.0.5

• Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• CraftBukkit/Spigot/Paper 1.12.2
• CraftBukkit 1.8.8
  Don't see how this could differ on other server versions.

Demonstration:
As an example here I have made it so that the first-join messages are sent to the staff channel. You would likely just want these in the primary channel though.

...
# To disable a message from showing, use 'none' as the channel name.
message-types:
  # Join messages sent when a player joins the Minecraft server.
  join: primary
  # Join messages sent when a player joins the Minecraft server for the first time.
  first-join: staff

...
  # This is the message sent to Discord when a player joins the minecraft server for the first time
  # The following placeholders can be used here:
  # - {username}: The name of the user joining
  # - {displayname}: The display name of the user joining
  # - {joinmessage}: The full default join message used in game
  # - {online}: The amount of players online
  # - {unique}: The amount of unique players to ever join the server
  # ... PlaceholderAPI placeholders are also supported here too!
  first-join: ":arrow_right:** {displayname} has joined the server for the first time!**"

[diademiemi]

Updated title, GitHub had changed it back when I switched branches

[JRoy]

just a few things otherwise good

[mdcfe]

Thanks for the contribution! Apologies for the delay in merging this.

[diademiemi]

No worries! I've got a few other Discord PRs open, there was one I was going to change something about which I honestly forgot about up until now, I'll get to working on that right now.