remove DiscordBM dependency

Sources/DiscordKit/Models.swift

@@ -23,6 +23,8 @@ public enum GatewayPayload: Decodable {
 public struct GatewayMessage: Decodable {
     let op: Int
     let d: GatewayPayload?
+    let s: Int?
+    let t: String?
 
     enum CodingKeys: String, CodingKey {
         case t
@@ -34,6 +36,8 @@ public struct GatewayMessage: Decodable {
     public init(from decoder: Decoder) throws {
         let container = try decoder.container(keyedBy: CodingKeys.self)
         op = try container.decode(Int.self, forKey: .op)
+        s = try container.decode(Int?.self, forKey: .s)
+        t = try container.decode(String?.self, forKey: .t)
         switch op {
             case 10:
                 let hello = try container.decode(HelloPayload.self, forKey: .d)