Add GitHub Actions workflow for Node.js with Webpack

[devops863]

Summary

Adds a GitHub Actions workflow to build the project using Node.js and Webpack.

Changes

• Add GitHub Actions workflow file
• Install dependencies in CI
• Run the Webpack build on pull requests / repository events

Purpose

This sets up basic CI so changes can be automatically validated and build issues can be caught earlier.

Notes

• This PR only adds the workflow configuration
• No application source code was changed

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.