[@mantine/dates] DateInput: Separate clearable and allowDeselect logic

[DanielGibbsNZ]

Fixes #4524.

The change is a bit larger than I intended but I believe it makes the clearable and allowDeselect props more understandable:

• clearable controls whether the DateInput can be cleared (via the text field or the date picker)
• allowDeselect controls whether clicking on the existing date in the date picker clears the field (only applicable if clearable is set), defaults to true if clearable is set

If this is too much of a change, left me know and I'll revert it back to my original idea of just allowing allowDeselect to be manually set. The downside with this approach is that it can result in some weirdly inconsistent behaviour where the value can be cleared by clicking the date when allowDeselect is set and clearable isn't set.

[rtivital]

Thanks!