chore: Add description (#11)

## Checklist

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- [X] Code conforms to the [Elixir
Styleguide](https://github.com/christopheradams/elixir_style_guide)

## Problem

Cannot publish to Hex without a description

## Details

Add a basic description for now.

mix.exs

@@ -5,6 +5,7 @@ defmodule Lunar.MixProject do
     [
       app: :lunar,
       name: "Lunar",
+      description: "A Lua runtime for Elixir built on Luerl",
       deps: deps(),
       elixir: "~> 1.13",
       elixirc_paths: elixirc_paths(Mix.env()),