feat(lib): cjs instead of umd as default format for multiple entries

[schummar]

Description

This is a follow up to #7047. I propose to use different default format settings when using multiple entries in lib mode.

At the moment using a config like this:

lib: {
  entry: {
      primary: 'src/index.ts',
      secondary: 'src/secondary.ts'
  }
}

will result in an error

Multiple entry points are not supported when output formats include "umd" or "iife".

That is because, if not specified otherwise, the output formats in lib mode will be ['es', 'umd'] and Rollup does not support multiple entry points for UMD (rollup/rollup#2072). Since this is not a nice user experience, I think it's better to not use UMD by default - if multiple entries are used. Instead CommonJs could be used, which is at least a part of UMD, minus the AMD part.

Additional context

Since multiple entries are not released yet, this is not a breaking change. If we can get it in before 3.2 😉

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

Thanks! This looks good to me.