Reduce Dependabot PR noise for NPM package ecosystem

To reduce the noise of too many PRs from NPM dependencies, where most of
them are only scoped for (local) development, two optimizations have
been made:

1. The schedule changed to the `monthly` interval [1].
   This is still enough to keep up with the fast updates in the NPM
   ecosystem.
2. Only watch production packages (`dependencies`) and ignore
   development packages (`devDependencies`).
   The packages used for local or CI/CD development purposes are not
   required to be the latest version just for the sake of being
   up-to-date without a specific need or benefit.

Since GitHub takes security really serious [2], important Dependabot
security updates [3] are triggered manually by a security advisor so
there is no risk of missing important versions bumps when reducing the
schedule interval.

  "Use the `allow` option to customize which dependencies are updated.
  This has no impact on security updates for vulnerable dependencies."

[1]: https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/configuration-options-for-dependency-updates#scheduleinterval
[2]: https://github.com/security
[3]: https://docs.github.com/en/free-pro-team@latest/github/managing-security-vulnerabilities/about-dependabot-security-updates

GH-65

.github/dependabot.yml

@@ -24,7 +24,9 @@ updates:
   - package-ecosystem: "npm"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
+    allow:
+      - dependency-type: "production"
     reviewers:
       - "svengreb"
     labels: