Read Elixir version from mix.exs file

In addition to the `.exenv-version` file, asdf-elixir should also be able to read the Elixir version out of a `mix.exs` file in the current directory.

See this related issue for kiex: https://github.com/taylor/kiex/pull/54

Start by examining how the plugin currently reads the .exenv-version file, then compare that behavior with the mix.exs file in the current directory. The change is done when the Elixir version can be read from mix.exs as well as .exenv-version, while existing version detection continues to work.

Written by the indexing model from the issue text.