Ignore major version updates for "node-fetch" (#164)

We don't have sufficient test coverage to know if version 3 is
a safe upgrade, so it would be nice to make the bot less noisy.

Author: miketaylr

.github/dependabot.yml

@@ -4,3 +4,7 @@ updates:
   directory: /
   schedule:
     interval: monthly
+  ignore:
+      - dependency-name: "node-fetch"
+        update-types: ["version-update:semver-major"]
+