Add Mix task to create a custom emoji list

lib/mix/tasks/create_custom_emoji_list.ex

@@ -0,0 +1,36 @@
+defmodule Mix.Tasks.CreateCustomEmojiList do
+  use Mix.Task
+
+  @emoji_root "priv/static/emoji/"
+  @custom_emoji_root "#{@emoji_root}custom"
+  @custom_emoji_file "config/custom_emoji.txt"
+
+  @shortdoc "Generates #{@custom_emoji_file}"
+  def run(_) do
+    if File.dir?(@custom_emoji_root) do
+      IO.puts("#{@custom_emoji_root} directory found.")
+      {:ok, file_handle} = File.open(@custom_emoji_file, [:write])
+      handle_dir(@custom_emoji_root, file_handle)
+      File.close(file_handle)
+    else
+      IO.puts("#{@custom_emoji_root} directory does not exist.")
+    end
+  end
+
+  def handle_dir(dir, file_handle) do
+    for entry <- File.ls!(dir) do
+      full_entry_path = Path.join(dir, entry)
+      if File.dir?(full_entry_path) do
+        handle_dir(full_entry_path, file_handle)
+      else
+        handle_file(full_entry_path, file_handle)
+      end
+    end
+  end
+
+  def handle_file(file, file_handle) do
+    relative_name = Path.relative_to(file, @emoji_root)
+    basename = String.downcase(Path.basename(file, Path.extname(file)))
+    IO.puts(file_handle, "#{basename}, /emoji/#{relative_name}")
+  end
+end