Set reviewer for all Dependabot PRs (#52)

This is to ensure that the core maintainer gets notified when Dependabot
creates a PR.

See:
https://docs.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates#reviewers

Author: johnboyes

.github/dependabot.yml

@@ -12,3 +12,5 @@ updates:
     labels:
       - "dependencies"
       - "patch"
+    reviewers:
+      - "johnboyes"