Add support for the abstract erlang frontend to `liblumen_eir_interpreter`

[hansihe]

Part of our strategy for supporting elixir is by using the mix compiler located in lumen_elixir (abstr_out branch).

The format this emits is abstract erlang (.abstr), different from the regular erlang code used by the normal frontend (.erl).

In libeir_syntax_erl there is a separate frontend for parsing and lowering .abstr files into eir.

This issue is for adding support for .abstr files to the CLI for liblumen_eir_interpreter, presumably checking the extension of the file, and calling the regular frontend for .erl files and the abstract erlang frontend for .abstr files.

Right now, handling of .erl files is done here in liblumen_eir_interpreter.

Feel free to contact me for mentoring.

[lawik]

I'll take a crack at it :)