Update dependabot config

Exclude more npm packages which we can't update pending
web-platform-tests#232, and
start updating GitHub Actions.

Author: gsnedders

.github/dependabot.yml

@@ -5,6 +5,16 @@ updates:
   schedule:
     interval: monthly
   ignore:
+      - dependency-name: "chai"
+        update-types: ["version-update:semver-major"]
+      - dependency-name: "eslint"
+        update-types: ["version-update:semver-major"]
+      - dependency-name: "flags"
+        update-types: ["version-update:semver-minor"]
       - dependency-name: "node-fetch"
         update-types: ["version-update:semver-major"]
 
+- package-ecosystem: github-actions
+  directory: /
+  schedule:
+    interval: monthly