build: add dependabot.yml file

legacy/.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 3
+updates:
+- package-ecosystem: npm
+  directory: "/"
+  schedule:
+    interval: monthly
+  open-pull-requests-limit: 10
+  allow:
+    - dependency-name: "*"
+      dependency-type: "all"