Add dependabot configuration

To prepare for release, we should ensure we keep our dependencies up to
date. We can use dependabot for this, and this configuration mimicks the
configuration we use in ably-js.

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "daily" # weekdays (Monday to Friday)
+    labels: [ ] # prevent the default `dependencies` label from being added to pull requests