Associate .elixir files with elixir-mode

core/prelude-packages.el

@@ -136,6 +136,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.dart\\'" dart-mode dart-mode)
     ("\\.ex\\'" elixir-mode elixir-mode)
     ("\\.exs\\'" elixir-mode elixir-mode)
+    ("\\.elixir\\'" elixir-mode elixir-mode)
     ("\\.erl\\'" erlang erlang-mode)
     ("\\.feature\\'" feature-mode feature-mode)
     ("\\.go\\'" go-mode go-mode)