Add dependabot (#826)

It has a low limit on how many PRs it's allowed to open at a time, so it
should not flood the repo.

.github/dependabot.yml

@@ -0,0 +1,14 @@
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 2
+    versioning-strategy: increase
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 2
+    versioning-strategy: increase