userdiff: add Elixir to supported userdiff languages

Adds support for xfuncref in Elixir[1] language which is Ruby-like
language that runs on Erlang[3] Virtual Machine (BEAM).

[1]: https://elixir-lang.org
[2]: https://www.erlang.org

Signed-off-by: Łukasz Niemier <lukasz@niemier.pl>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/elixir-protocol-implementation

@@ -0,0 +1,5 @@
+defimpl RIGHT do
+  # Docs
+  # Docs
+  def foo(ChangeMe), do: :ok
+end