Enable dependabot

Signed-off-by: Christopher Ng <chrng8@gmail.com>

.github/dependabot.yml

+version: 2
+updates:
+  - package-ecosystem: cargo
+    directory: "/"
+    schedule:
+      interval: weekly
+      day: tuesday
+      time: "02:00"
+    open-pull-requests-limit: 10