[Feature] Missing index file in dayjs/plugin

**Describe the bug**

Hello,

when importing plugins i should able to do :

```
import {utc, timezone, localizedFormat} from 'dayjs/plugin';
```
i does not work because there is no index.js in dayjs/plugin directory

```
import utc from './utc';
import timezone from './timezone';
import localizedFormat from './localizedFormat';
[...]

export {
utc,
timezone,
localizedFormat
[...]
}
```

**Expected behavior**

plugins are importing correclty

**Information**
- Day.js Version 1.10.4
- OS: Ubuntu
- Browser chrome 89

there is no feature request template so i'm using the bug one.

thanks.

Start in the dayjs/plugin directory and inspect the existing plugin modules referenced in the issue. Add the package entry point described there so named imports such as utc, timezone, and localizedFormat work, then verify the example import resolves successfully.

Written by the indexing model from the issue text.