Add pre-defined event for the player discovering a rare

This is similar to adding a chat trigger for `$NAME has discovered` but is built-in (since it's such a common thing players want) rather than requiring the user to create a chat trigger with a custom regex.

PluginCore.Events.cs

@@ -17,6 +17,7 @@ private void CharacterFilter_LoginComplete(object sender, EventArgs e)
                 Core.CharacterFilter.Death += CharacterFilter_Death;
 
                 TriggerWebhooksForEvent(EVENTS.LOGIN, Core.CharacterFilter.Name + " has logged in.");
+                ChatPatterns.Add(new ChatPattern(EVENTS.RARE, Core.CharacterFilter.Name + " has discovered "));
             }
             catch (Exception ex) { Util.LogError(ex); }
         }

PluginCore.MainView.cs

@@ -536,6 +536,7 @@ private void PopulateEventChoices()
                 chcEventTriggerEvent.Add(EVENTDESC.LEVEL, EVENTS.LEVEL);
                 chcEventTriggerEvent.Add(EVENTDESC.DEATH, EVENTS.DEATH);
                 chcEventTriggerEvent.Add(EVENTDESC.DROPONDEATH, EVENTS.DROPONDEATH);
+                chcEventTriggerEvent.Add(EVENTDESC.RARE, EVENTS.RARE);
             }
             catch (Exception ex)
             {

PluginCore.cs

@@ -31,6 +31,7 @@ public struct EVENTS {
             public const string LEVEL = "LEVEL";
             public const string DEATH = "DEATH";
             public const string DROPONDEATH = "DROPONDEATH";
+            public const string RARE = "RARE";
         };
 
         public struct EVENTDESC
@@ -40,6 +41,7 @@ public struct EVENTDESC
             public const string LEVEL = "You level up";
             public const string DEATH = "You die";
             public const string DROPONDEATH = "You drop items on death";
+            public const string RARE = "You find a rare";
         }
 
         // Just GameEvent events the plugin handles