feat: show current queue

When given a `/queue show` command, Sergei will print out a list of
songs in the current queue.  As this only outputs the URLs, we rely on
Discord to add the embeds and therefore title of the tracks.

lib/sergei/commands/queue.ex

@@ -11,7 +11,8 @@ defmodule Sergei.Commands.Queue do
 
   @queue_commands [
     opt.(1, "add", "Add a song to the queue", options: @queue_add_opts),
-    opt.(1, "clear", "Clear the queue", [])
+    opt.(1, "clear", "Clear the queue", []),
+    opt.(1, "show", "Show the queue", [])
   ]
 
   def subcommands() do
@@ -43,4 +44,22 @@ defmodule Sergei.Commands.Queue do
         {:ok, "I'm not playing anything right now."}
     end
   end
+
+  def handle(guild_id, "show", _opts) do
+    case Sergei.Player.queue_list(guild_id) do
+      :not_playing ->
+        {:ok, "I'm not playing anything right now."}
+
+      [] ->
+        {:ok, "The queue is empty."}
+
+      songs_in_queue ->
+        song_list =
+          songs_in_queue
+          |> Enum.with_index(1)
+          |> Enum.reduce("", fn {song, idx}, acc -> acc <> "\n#{idx}. #{song}" end)
+
+        {:ok, song_list}
+    end
+  end
 end

lib/sergei/player.ex

@@ -34,6 +34,11 @@ defmodule Sergei.Player do
     GenServer.call(__MODULE__, {:queue_clear, guild_id})
   end
 
+  @spec queue_list(integer()) :: [String.t()] | :not_playing
+  def queue_list(guild_id) do
+    GenServer.call(__MODULE__, {:queue_list, guild_id})
+  end
+
   @spec pause(integer()) :: :ok | :not_playing | {:error, String.t()}
   def pause(guild_id) do
     GenServer.call(__MODULE__, {:pause, guild_id})
@@ -173,6 +178,19 @@ defmodule Sergei.Player do
     }
   end
 
+  # Queue List
+  @impl true
+  def handle_call({:queue_list, guild_id}, _from, state) do
+    {
+      :reply,
+      state
+      |> Map.get(guild_id)
+      |> Map.get(:queue)
+      |> :queue.to_list(),
+      state
+    }
+  end
+
   # Pause
   @impl true
   def handle_call({:pause, guild_id}, _from, state) do