Move the Apps JavaScript bundles from WebPack to Rollup

[Filed by Copilot on behalf of @bghgary]

`Apps/UnitTests/JavaScript/webpack.config.js` and `Apps/PrecompiledShaderTest/JavaScript/webpack.config.js` are the repo's only bundler configs. WebPack was chosen to align with Babylon.js, which has since moved to Rollup.

Raised by @ryantrem reviewing #1733:

> Not for this PR, but we made this WebPack to align with BJS, but BJS has moved to Rollup, so we really should do the same here.

Both should move together rather than leaving the repo split across two bundlers.

Start by reading Apps/UnitTests/JavaScript/webpack.config.js and Apps/PrecompiledShaderTest/JavaScript/webpack.config.js to understand the two existing bundle configurations. Migrate both together and verify that the Apps JavaScript bundles use Rollup rather than leaving either WebPack configuration in the repository.

Written by the indexing model from the issue text.