Migrate from handlebars to simple string concats

[JReinhold]

handlebars is part of @storybook/core despite only being used in a single place in builder-webpack5:

1. https://github.com/storybookjs/storybook/blob/next/code/builders/builder-webpack5/src/preview/virtual-module-mapping.ts/#L54-L66
2. https://github.com/storybookjs/storybook/blob/next/code/builders/builder-webpack5/templates/virtualModuleModernEntry.js.handlebars

Our use case for it is so simple that builder-vite just manually builds the string with simple template strings: https://github.com/storybookjs/storybook/blob/next/code/builders/builder-vite/src/codegen-modern-iframe-script.ts

We should migrate the Webpack builder to do the same, and get rid of handlebars altogether.