feat(exports): add extensions option for subpath export keys

[SinhSinhAn]

Summary

Adds a new jsExtension boolean option to ExportsOptions that appends .js extensions to all subpath export keys (except the root "."), following the Node.js recommendation for subpath exports.

Before (jsExtension: false or unset):

{
  "exports": {
    ".": "./index.js",
    "./utils": "./utils.js"
  }
}

After (jsExtension: true):

{
  "exports": {
    ".": "./index.js",
    "./utils.js": "./utils.js"
  }
}

Changes

• Added jsExtension?: boolean to ExportsOptions interface with JSDoc documentation
• Implemented the extension logic in the export key generation loop (only affects non-root keys)
• Works correctly with all existing features: dual formats, directory index entries, devExports, customExports

Tests

Added 5 new test cases covering:

• Basic subpath export keys with .js extension
• Directory index entries (e.g., ./foo.js from foo/index.js)
• Dual format (ESM + CJS) with jsExtension
• Root export is unaffected
• Interaction with devExports and publishExports

All existing tests continue to pass (40 total).

Closes #898

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commit: 51f69aa

[sxzz]

please rename jsExtension to extensions