Add minimal `mix.exs`

Add minimal `mix.exs`

This should please mix(1) which should be able to build recon without
fetching rebar3 (even though it is already available system-wide).

Here is the failure reported by mix(1) otherwise in a unattended build:

    Shall I install rebar3? (if running non-interactively, use "mix local.rebar --force") [Yn]
    ** (Mix) Could not find "rebar3" to compile dependency :recon, please ensure "rebar3" is available

In `src/recon.app.src`, we also record `mix` as an acceptable build
tool. We also ensure `mix.exs` is uploaded to Hex.pm.