Set up dependabot config (#1123)

Summary:
Probably needs some further configuring to match our current ignores, but let's see.
Pull Request resolved: https://github.com/facebook/flipper/pull/1123

Reviewed By: mweststrate

Differential Revision: D21402227

Pulled By: passy

fbshipit-source-id: 001c4c8e393dfe1e6d8838b0f43c7e6c8cf555db

.dependabot/config.yml

@@ -0,0 +1,8 @@
+version: 1
+update_configs:
+  - package_manager: "javascript"
+    directory: "/desktop"
+    update_schedule: "weekly"
+  - package_manager: "rust:cargo"
+    directory: "/packer"
+    update_schedule: "weekly"