Why does locale/en.js not have `formats`?

The `en` locale does not have `formats` and similar properties required by the `localeData` plugin - unlike other locales.

This breaks the `dayjs.localeData().longDateFormat` method, which [expects the loaded locale to have a `formats` property](https://github.com/iamkun/dayjs/blob/dev/src/plugin/localeData/index.js#L17)

Wouldn't it make sense to add [these formats](https://github.com/iamkun/dayjs/blob/8a0811226679a6128714a0ed188f2d0ef0254313/src/plugin/localizedFormat/utils.js#L5) to `en.js`?

Start with locale/en.js and inspect how src/plugin/localeData/index.js reads the locale's formats property. Compare the expected values with the formats referenced in src/plugin/localizedFormat/utils.js. Done means the en locale supplies the required formats and localeData().longDateFormat works without the reported breakage.

Written by the indexing model from the issue text.