customParseFormat error only in production

**Describe the bug**
Production build throws the following error. However, it works fine locally. This package is being used in [excelJs](https://github.com/exceljs/exceljs) as a dependency. I am not using it directly in my project.

```
Application has thrown an uncaught exception and is terminated:TypeError: Cannot set property 'customParseFormat' of undefinedat D:\home\site\wwwroot\node_modules\dayjs\plugin\customParseFormat.js:1:2697
```

**Information**
- Day.js Version - dayjs@1.10.7
- OS: [Microsoft Windows [Version 10.0.14393]

Start with plugin/customParseFormat.js and compare how it is loaded in the local and production builds. Reproduce the reported TypeError with Day.js 1.10.7 on the stated Windows environment, considering its use through ExcelJS. Done means the production build loads customParseFormat without the reported error.

Written by the indexing model from the issue text.