The builder depends on internal use of handlebars that will be removed in Storybook 8.4

[JReinhold]

For Storybook 8.4 we're doing a lot of internal cleanup and optimisation (see storybookjs/storybook#29038).

As part of my work to replace the express dev-server with polka, I wanted to try out if that worked with RSBuild, because the server is exposed to the builders via the start() function. However I never got that far, because RSBuild fails with the following error in the latest alpha releases of Storybook 8.4.0:

=> Failed to build the preview
TypeError: (0 , import_common.handlebars) is not a function
    at getVirtualModules (./node_modules/storybook-builder-rsbuild/dist/index.js:195:66)
    at async iframe_rsbuild_config_default (./node_modules/storybook-builder-rsbuild/dist/index.js:371:72)
    at async rsbuild (./node_modules/storybook-builder-rsbuild/dist/index.js:656:23)
    at async Module.start (./node_modules/storybook-builder-rsbuild/dist/index.js:691:18)
    at async storybookDevServer (./node_modules/@storybook/core/dist/core-server/index.cjs:37195:11)
    at async buildOrThrow (./node_modules/@storybook/core/dist/core-server/index.cjs:36205:12)
    at async buildDevStandalone (./node_modules/@storybook/core/dist/core-server/index.cjs:38385:78)
    at async withTelemetry (./node_modules/@storybook/core/dist/core-server/index.cjs:36950:12)
    at async dev (./node_modules/@storybook/core/dist/cli/bin/index.cjs:2877:3)
    at async r.<anonymous> (./node_modules/@storybook/core/dist/cli/bin/index.cjs:2929:74)

This is because we've removed handlebars from the Storybook core/common exports, as it was unnecessary. It was never intended to be used as a public API, but it's fair that you perhaps just copied the logic from the Webpack builder.

You should be able to to the same migration that @ndelangen we did for the Webpack builder in storybookjs/storybook#29208. High-level, that PR:

1. converts the handlebar template to a regular JS file
2. uses '{{someVariable}}' for stuff that needs replacement
3. Uses regular replaceAll() calls to do the variable replacements.

[fi3ework]

Thanks @JReinhold for the kind remind. I'll do the same migration quickly.