fix: range not selected when missing the from value

[Sam-Apostel]

Context

A user might want to filter all items before a specific date and modify the selected state outside react-day-picker.
While this works perfectly when only from is set. But when only to is set, the calendar doesn't reflect this state.
The next time a user selects a date, the state is affected like when no values were set at all.
Discussion: #1875

Analysis

selected on a range picker accepts a DateRange where from is undefined but to is a Date.
Since this works in one direction and the types allow this state, this component should reflect the other direction too.

Solution

To prevent current behaviour changes, All of the logic additions only take effect when to is set and from is not.
From within this component, this is an unreachable state, but when the state is controlled externally, it is now reflected in the internal state.

All other code changes and restructuring were made to benefit typescript.
No test were changed, few were added for specific state.

[gpbl]

Ah, I knew there was a case I missed there. Thanks a lot for your PR, the tests and the fix. It doesn't happen often to get an high quality contribution!

[gpbl]

This fix has been released as https://github.com/gpbl/react-day-picker/releases/tag/v8.8.1. (Not sure why the release doesn't appear here)