Add .d.ts

[yuki0410-dev]

Thanks for the continuous OSS development.

Is your feature request related to a problem? Please describe.
I use react-datepicker along with typescript in my work. However, @types/react-datepicker is currently in a separate repository, so new features of react-datepicker are not immediately available when they are released.

Describe the solution you'd like
I would like to distribute d.ts files in this repository so that the latest react-datepicker functionality is available in typescript without @types/react-datepicker.

Describe alternatives you've considered
Wait for the latest @types/react-datepicker to be released. Or create a PR.
We also considered rewriting this entire repository to TS, but due to the scale of the project, we felt it would not be practical.

Additional context
We need to introduce a check of d.ts by CI with reference to @types/react-datepicker.

[yuki0410-dev]

refs #4000

[yuki0410-dev]

refs #4700