Add /slay alias for /kill

Like all of the other server mods I've used use /slay and not /kill and
it's really frustrating to type the wrong thing so I'm just going to add
an alias and hope nobody else minds.

CHANGELOG.md

@@ -14,6 +14,7 @@ This is the rolling changelog for TShock for Terraria. Use past tense when addin
 
 ## Upcoming changes
 * Changed the world autosave message so that it no longer warns of a "potential lag spike." (@hakusaro)
+* Added `/slay` as an alias for `/kill` to be more consistent with other server mods. (@hakusaro)
 
 ## TShock 4.5.4
 * Fixed ridiculous typo in `GetDataHandlers` which caused TShock to read the wrong field in the packet for `usingBiomeTorches`. (@hakusaro, @Arthri)

TShockAPI/Commands.cs

@@ -549,7 +549,7 @@ namespace TShockAPI
 			{
 				HelpText = "Heals a player in HP and MP."
 			});
-			add(new Command(Permissions.kill, Kill, "kill")
+			add(new Command(Permissions.kill, Kill, "kill", "slay")
 			{
 				HelpText = "Kills another player."
 			});