Don't break builds of our dependents if new warnings appear

We limit warnings-as-errors to the Mix test env, which still ensures
tests will fail (and CI) if there are warnings.

GitHub issue:
* #7

CHANGELOG.md

@@ -11,6 +11,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - `new!/0` and `new!/1` to ease storing context in module attributes
 
+### Fixed
+
+- unwarranted risk of new future warnings breaking the builds of sqids
+  dependents
+
 ## [0.1.0] - 2023-10-28
 
 ### Added

mix.exs

@@ -5,17 +5,17 @@ defmodule Sqids.MixProject do
   @source_url "https://github.com/sqids/sqids-elixir"
 
   def project do
+    mix_env = Mix.env()
+
     [
       app: :sqids,
       version: @version,
       description: description(),
       elixir: "~> 1.7",
-      start_permanent: Mix.env() === :prod,
+      start_permanent: mix_env === :prod,
       deps: deps(),
-      elixirc_paths: elixirc_paths(Mix.env()),
-      elixirc_options: [
-        warnings_as_errors: true
-      ],
+      elixirc_paths: elixirc_paths(mix_env),
+      elixirc_options: elixirc_options(mix_env),
       docs: docs(),
       test_coverage: [
         summary: [
@@ -75,6 +75,14 @@ defmodule Sqids.MixProject do
     end
   end
 
+  defp elixirc_options(env) do
+    if env === :test do
+      [warnings_as_errors: true]
+    else
+      []
+    end
+  end
+
   defp docs do
     [
       main: "readme",