enable GitHub Dependabot

this ensures that the dependencies are kept up to date. see [the docs][]
for further information.

[the docs]: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates

Author: rursprung

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"