Imports in NPM Packges broken (missing js extension)

**Describe the bug**
I try to load dayjs directly as esm module via a importmap.
This is not possible, cause the imports in the npm module ar emissing the .js extensions

**Expected behavior**
all imports in javascript should contain extensions

**Information**
- Day.js Version : newest

Inspect the published Day.js npm package and its ESM import statements first, then compare them with the source or build output. The issue is done when JavaScript imports in the package include .js extensions and the package loads through an import map.

Written by the indexing model from the issue text.