Add Join and Play handlers, Expand Command Builder functionality

Bot/Handler/MusicPlayer/JoinCommand/JoinHandler.cs

@@ -0,0 +1,34 @@
+using Discord;
+using Discord.WebSocket;
+using MediatR;
+using Victoria.Node;
+
+namespace Lunaris2.Handler.MusicPlayer.JoinCommand;
+
+public record JoinCommand(SocketSlashCommand Message) : IRequest;
+
+public class JoinHandler : IRequestHandler<JoinCommand>
+{
+    private readonly LavaNode _lavaNode;
+    private readonly DiscordSocketClient _client;
+    
+    public JoinHandler(LavaNode lavaNode, DiscordSocketClient client)
+    {
+        _lavaNode = lavaNode;
+        _client = client;
+    }
+    
+    public async Task Handle(JoinCommand command, CancellationToken cancellationToken)
+    {
+        var context = command.Message;
+        
+        await _lavaNode.EnsureConnected();
+
+        if (_lavaNode.HasPlayer(context.GetGuild(_client))) {
+            await context.RespondAsync("I'm already connected to a voice channel!");
+            return;
+        }
+        
+        await context.JoinVoiceChannel(_lavaNode);
+    }
+}