Add Dependabot config

Based on the template in testdouble/.github.

Author: rosston

.github/dependabot.yml

@@ -0,0 +1,14 @@
+# Please see the documentation for all configuration options:
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule: { interval: monthly }
+    cooldown: { default-days: 14 }
+    groups: { github-actions: { patterns: ["*"] } }
+  - package-ecosystem: npm
+    directory: "/"
+    schedule: { interval: monthly }
+    cooldown: { default-days: 14 }