Add Elixir code formatter

[maymillerricci]

This does the following to configure the Elixir code formatter for use here:

• adds a .formatter file so we can run mix format
• adds all the changes from running mix format
• adds a .tool-versions file so contributors can use consistent versions of Erlang and Elixir
• bumps the Elixir version on CI to match
• adds the check for formatted files to run on CI
• adds info to the Contributing section of the README about formatting

[germsvel]

Looks good! Thanks for adding this.