ci: add GitHub Dependabot to keep GitHub Actions up-to-date (#517)

> [GitHub] Actions are often updated with bug fixes and new features to
> make automated processes more reliable, faster, and safer. When you
> enable Dependabot version updates for GitHub Actions, Dependabot will
> help ensure that references to actions in a repository's workflow.yml
> file and reusable workflows used inside workflows are kept up to date.
>
> [...]
>
> If a more recent version of the action is available, Dependabot will
> send you a pull request that updates the reference in the workflow
> file to the latest version.
>
> -- GitHub Docs
>    https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

Link: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#commit-message
Link: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
Link: https://github.com/danth/stylix/pull/517

Co-authored-by: Daniel Thwaites <danthwaites30@btinternet.com>

.github/dependabot.yml

@@ -0,0 +1,13 @@
+---
+version: 2
+
+updates:
+  - package-ecosystem: github-actions
+
+    commit-message:
+      prefix: ci
+
+    directory: /
+
+    schedule:
+      interval: daily