modules/js/perf/perf_helpfunc.js and target tests all contain const isNodeJs, leading to re-definition when using associated *.html files:

	//... script above
     <script src="https://cdnjs.cloudflare.com/ajax/libs/benchmark/2.1.4/benchmark.js"></script>$
     <script src="../../opencv.js" type="text/javascript"></script>$
     <script src="../base.js"></script>$
     <script src="../perf_helpfunc.js"></script>$
     <script src="./perf_gaussianBlur.js"></script>$
   </body>$
</html>$

perf_helpfunc.js was imported (which globally defined const isNodeJs) and then perf_gussianBlur.js was imported (which globally defined const isNodeJs). Changing them to var resolves scoping issues when running in browser, while maintaining nodejs support.

Special thanks to https://github.com/youar for supporting this work; please inform if applying a copyright-header is appropriate attribution.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders=Custom
build_image:Docs=docs-js
build_image:Custom=javascript
buildworker:Custom=linux-1,linux-4,linux-f1