fix: set filename dynamically using a function

[Timeless0911]

Summary

In composeAutoExtensionConfig fuction, we should only return config about output.filename.js instead of whole output.filename object to avoid being merged into an array which Rsbuild will throw an error, see #911 (comment).

BTW, just record this scene in this PR that Rslib support setting function for output.filename.js and it will take effect in actual output. But we can not get correct js extension when we redirect js path in bundleless mode and will fallback to the default js extension (related to autoExtension config) which may not be consistent with the actual output.

Eg.

// config
output: {
  filename: {
    js: () => {
      return '[name].mjs';
    },
  },
},

// source
import { foo } from './foo';

// output
import * as __WEBPACK_EXTERNAL_MODULE__foo_js_fdf5aa2d__ from "./foo.js";

The actual output is foo.mjs but we can not get it and redirect to foo.js as a fallback.

So if users want to set function for output.filename.js, the [ext] should better be equal to the default js extension (related to autoExtension config).

Related Links

fix: #911

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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