ci: enable dependabot (#87)

On a monthly schedule.

Ref: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference#schedule-

Author: vszakats

.github/dependabot.yml

@@ -0,0 +1,9 @@
+# Copyright (C) Viktor Szakats. See LICENSE.md
+# SPDX-License-Identifier: MIT
+
+version: 2
+updates:
+  - package-ecosystem: 'github-actions'
+    directory: '/'
+    schedule:
+      interval: 'monthly'