Use dependabot to automatically update gh actions

Updating npm packages is for now disabled due to possible issues with lerna

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+###   Issues with lerna stop us from auto-updating npm dependencies
+###   See: https://github.com/codeoverflow-org/nodecg-io/issues/263
+# - package-ecosystem: "npm"
+#   directory: "/"
+#   schedule:
+#     interval: "daily"