Add npm ecosystem to Dependabot configuration

Follow-up to modelcontextprotocol#2168 (review)

Configure Dependabot to check npm dependencies in the repository
and create weekly update pull requests.

Author: koic

.github/dependabot.yml

@@ -4,3 +4,8 @@ updates:
     directory: '/'
     schedule:
       interval: 'weekly'
+
+  - package-ecosystem: 'npm'
+    directory: '/'
+    schedule:
+      interval: 'weekly'