Upgrade CI's clippy to giraffate/clippy-action.

This action integrates Clippy with reviewdog for an "improved code
review experience."

.github/workflows/main.yml

@@ -6,7 +6,7 @@ on:
     branches:
     - master
 
-name: Continuous integration
+name: Continuous Integration
 
 jobs:
   check:

.github/workflows/pr_review.yml

@@ -0,0 +1,14 @@
+name: PR Review
+on: [pull_request]
+jobs:
+  clippy:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: dtolnay/rust-toolchain@stable
+      with:
+        components: clippy
+    - uses: giraffate/clippy-action@v1
+      with:
+        reporter: 'github-pr-review'
+        github_token: ${{ github.token }}