Core: Fix manager.js ignored when sideEffects:false in package.json

[ndelangen]

Closes #

What I did

Add esbuild config to ignore side-effect:false in the user's package.json

see: https://esbuild.github.io/api/#ignore-annotations

How to test

• Add a manager.js file to a project using storybook
• Add a console.log('HELLO') in this manager.js
• Set side-effects: false in package.json
• start storybook
• expect to see "HELLO" in the console in the manager frame