Support compilation of Elixir files within a Gleam project

[lpil]

We can shell out and call elixirc?

We will need to do this last as the Elixir code may need to execute Gleam and Erlang code during compilation. How do we make it avaliable?

[tynanbe]

I think if we copy .ex files in the same fn where "native" modules get copied, we then need to use that new maybe_link_elixir_libs fn, keep a separate list of .ex files, and see if running this at the end of the main gleam@@compile.erl script would do the trick:

'Elixir.Kernel.ParallelCompiler':compile_to_path