Package doesn't define module entry points

**Describe the bug**
The package.json does not define `exports` as module entry points for better ESM support, see also: https://nodejs.org/api/packages.html#package-entry-points

**Expected behavior**
The build process exports ES modules, but they are not properly exposed. This is particularly important when you want to service ESM in a browser.

The following addition to the `package.json` file should sove the issue:

```json
{
"exports": {
".": "./esm/index.js",
"./utils": "./esm/utils.js",
"./contant": "./esm/constant.js",
"./locale/*": "./esm/locale/*.js",
"./plugin/*": "./esm/plugin/*.js",
}
}
```

**Information**
- Day.js Version latest
- OS: any
- Browser any
- Time zone: 'Europe/Berlin'

Start with package.json and inspect the build process outputs under esm/, especially index.js, utils.js, constant.js, locale files, and plugin files. Compare the package entry points with the Node.js package exports documentation and verify that the proposed subpaths resolve correctly for ESM consumers; done means the built modules are properly exposed without breaking existing package access.

Written by the indexing model from the issue text.