Add dependabot.yml.

Inspired by template at https://github.com/actions/typescript-action/blob/2e5a157793be8c19e8aa3f09bee02fc4d97b236c/.github/dependabot.yml

.github/dependabot.yml

@@ -0,0 +1,26 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    groups:
+      actions-minor:
+        update-types:
+          - minor
+          - patch
+
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: monthly
+    groups:
+      npm-development:
+        dependency-type: development
+        update-types:
+          - minor
+          - patch
+      npm-production:
+        dependency-type: production
+        update-types:
+          - patch