feat: support advanced entrypoints

[CyberAP]

Description 📖

This change adds support for more advanced entrypoints:

1. Entrypoints with custom names that don't match the file name
2. CSS-only entrypoints

Example config for custom names:

{
  build: {
    rollupOptions: {
      input: {
        'custom-name': '/some-other-file.js'
      }
    }
  }
}

Example config for CSS only entrypoint:

{
  build: {
    rollupOptions: {
      input: {
        'my-style': '/some-style.scss'
      }
    }
  }
}

Background 📜

At GitLab we have entrypoints that don't necessarily match the filenames. This is due to how we change the entrypoints based on the target release: community edition (CE) or enterprise (EE). EE entrypoints are resolved outside of the sourceCodeDir and thus don't match exactly the filename. Also, we use special names for those: pages/foo/bar/index.js will get a name pages.foo.bar. As you can see it can not be directly matched to the filename. That's why we need to respect the name property stored in the manifest and use it instead or the actual filename to resolve an asset.

The same thing applies to the stylesheets, we could have different name for these. So we have to resolve CSS entrypoints using names property, which as added in vitejs/vite#19912 and released in https://github.com/vitejs/vite/releases/tag/v7.0.0-beta.0.

The Fix 🔨

We fix the issue by using name and names property for entries with isEntry field. For such entries we create new records in the manifest with name (or each key in names) used as a primary key.

[CyberAP]

Some corrections need to be made after vitejs/vite#20585. Setting to draft for now.

[CyberAP]

I've talked to the Vite/Rolldown team and turns out both name and names fields must be kept. These fields will be present in both Vite 7 and Vite 8. So this requires no changes, undrafting now.

[ElMassimo]

Any reason not to point to the same entry object instead of creating a new one?

[ElMassimo]

Adding .js feels like a hack to make using vite_javascript_tag more convenient.

[ElMassimo]

Something that seems fragile is assuming that name is used for JS entrypoints while names is used for CSS entrypoints. While that may hold true in the current version of Vite, it seems more of an implementation detail than an intentional contract.

Is the Vite dev server able to resolve custom entrypoint names in this fashion? In particular, custom name + .js.
Does it serve CSS assets requested by their custom name with appropriate content headers?