Add diff-check to catch missing Appraisal runs

If we forget to run `bundle exec appraisal`, which is not uncommon, it
means we miss when the appraisal files change. This should catch it and
fail the build, reporting back that they changed and hinting at what
should be done.

https://github.com/nickcharlton/diff-check
https://nickcharlton.net/posts/diff-check-github-action

.github/workflows/diff-check.yml

@@ -0,0 +1,16 @@
+---
+name: diff-check
+on: [push]
+
+jobs:
+  appraisal:
+    runs-on: ubuntu-latest
+    env:
+      DIFF_CHECK_APPRAISAL: "true"
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+      - run: bundle install
+      - uses: nickcharlton/diff-check@main
+        with:
+          command: bundle exec appraisal

Gemfile

@@ -1,5 +1,5 @@
 source "https://rubygems.org"
-ruby "3.2.2" unless ENV["CI"]
+ruby "3.2.2" unless ENV["CI"] && !ENV["DIFF_CHECK_APPRAISAL"]
 
 gemspec