add player package with key client-side player functionality, update protocol codecs, and refactor metadata definitions and slot usage

pkg/game/player/event.go

@@ -0,0 +1,11 @@
+package player
+
+import "github.com/Tnze/go-mc/chat"
+
+type MessageEvent struct {
+	Message chat.Message
+}
+
+func (m MessageEvent) EventID() string {
+	return "player:message"
+}