build(deps-dev): bump custom-elements-manifest in /plugins/dev-helpers

Bumps [custom-elements-manifest](https://github.com/webcomponents/custom-elements-manifest) from 1.0.0 to 2.1.0.
- [Release notes](https://github.com/webcomponents/custom-elements-manifest/releases)
- [Changelog](https://github.com/webcomponents/custom-elements-manifest/blob/main/CHANGELOG.md)
- [Commits](https://github.com/webcomponents/custom-elements-manifest/commits/v2.1.0)

---
updated-dependencies:
- dependency-name: custom-elements-manifest
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

plugins/dev-helpers/package.json

@@ -14,6 +14,6 @@
   "types": "lib/src/index.d.ts",
   "devDependencies": {
     "@inventage-web-components/common": "*",
-    "custom-elements-manifest": "^1.0.0"
+    "custom-elements-manifest": "^2.1.0"
   }
 }