dayjs.extend() and dayjs.locale() result in an error when dayjs is imported from ESM bundle

**Describe the bug**

Extending with plugins and setting locale doesn't seem to work when using ESM bundle.
Following code produces following errors when used in Angular Typescript project.
```ts
import * as dayjs from 'dayjs/esm';
import 'dayjs/esm/locale/en-gb';
import * as customParseFormat from 'dayjs/esm/plugin/customParseFormat';

dayjs.locale('en-gb');
dayjs.extend(customParseFormat);
```
```
"export 'extend' (imported as 'dayjs') was not found in 'dayjs/esm'
"export 'locale' (imported as 'dayjs') was not found in 'dayjs/esm'
```

The code above works fine when importing directly from `dayjs` instead of `dayjs/esm`.

**Information**
- Day.js Version 1.9.6
- Angular v10

Reproduce the Angular v10 TypeScript example using the dayjs/esm entry point, the en-gb locale, and the customParseFormat plugin. Inspect the ESM bundle exports first; done means dayjs.locale() and dayjs.extend() work without the reported missing-export errors, while the regular dayjs import remains unaffected.

Written by the indexing model from the issue text.