feat: add an action that handles git checkouts for all workflows

Author: samrose

.github/actions/shared-checkout/action.yml

@@ -0,0 +1,12 @@
+name: Checkout
+description: Checkout repository for pull requests and branches
+runs:
+  using: "composite"
+  steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+      with:
+        repository: ${{ github.event.pull_request.head.repo.full_name || github.repository }}
+        ref: ${{ github.event.pull_request.head.sha || github.sha }}
+        fetch-depth: 0
+        fetch-tags: true