Add :leex to Mix compilers

[wojtekmach]

Before this patch on Elixir main:

~/src/floki[main]% mix compile --force
warning: in order to compile .xrl files, you must add "compilers: [:leex] ++ Mix.compilers()" to the "def project" section of your mix.exs
  (mix 1.17.0-dev) lib/mix/tasks/compile.leex.ex:69: Mix.Tasks.Compile.Leex.preload/1
  (mix 1.17.0-dev) lib/mix/compilers/erlang.ex:66: Mix.Compilers.Erlang.compile/6
  (mix 1.17.0-dev) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5
  (mix 1.17.0-dev) lib/mix/tasks/compile.all.ex:124: Mix.Tasks.Compile.All.run_compiler/2
  (mix 1.17.0-dev) lib/mix/tasks/compile.all.ex:104: Mix.Tasks.Compile.All.compile/4
  (mix 1.17.0-dev) lib/mix/tasks/compile.all.ex:93: Mix.Tasks.Compile.All.with_logger_app/2

Compiling 1 file (.xrl)
Compiling 2 files (.erl)
Compiling 31 files (.ex)
Generated floki app

[philss]

@wojtekmach thank you! 💜