Move the Apps JavaScript bundles from WebPack to Rollup

[bghgary]

[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.

[bghgary]

Should do the same to JsRuntimeHost also.