add github workflow to cargo update (#64)

.github/workflows/dependencies.yml

@@ -0,0 +1,66 @@
+# Runs `cargo update` periodically.
+
+name: Update Dependencies
+
+on:
+  schedule:
+    # Run weekly
+    - cron: 0 0 * * SUN
+  workflow_dispatch:
+    # Needed so we can run it manually
+
+env:
+  GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  BRANCH: cargo-update
+  TITLE: "chore(deps): weekly `cargo update`"
+  BODY: |
+    Automation to keep dependencies in `Cargo.lock` current.
+
+    <details><summary><strong>cargo update log</strong></summary>
+    <p>
+
+    ```log
+    $cargo_update_log
+    ```
+
+    </p>
+    </details>
+
+permissions:
+  checks: write
+  contents: write
+
+jobs:
+  update:
+    name: Update
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+
+      - name: cargo update
+        # Remove first line that always just says "Updating crates.io index"
+        run: |
+          cargo update --color never 2>&1 | sed '/crates.io index/d' | tee -a cargo_update.log
+
+      - name: craft commit message and PR body
+        id: msg
+        run: |
+          export cargo_update_log="$(cat cargo_update.log)"
+
+          echo "commit_message<<EOF" >> $GITHUB_OUTPUT
+          printf "$TITLE\n\n$cargo_update_log\n" >> $GITHUB_OUTPUT
+          echo "EOF" >> $GITHUB_OUTPUT
+
+          echo "body<<EOF" >> $GITHUB_OUTPUT
+          echo "$BODY" | envsubst >> $GITHUB_OUTPUT
+          echo "EOF" >> $GITHUB_OUTPUT
+
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@v7
+        with:
+          add-paths: ./Cargo.lock
+          commit-message: ${{ steps.msg.outputs.commit_message }}
+          title: ${{ env.TITLE }}
+          body: ${{ steps.msg.outputs.body }}
+          branch: ${{ env.BRANCH }}