feat: Add support for updating a range without losing the opposite end of the selection

[GCastilho]

Implemented the 'keepBoundarySelected' prop, allowing users to "update" a range by clicking on one of its ends.

This new property configures whether the range should remain selected if the user clicks on one of the "range boundaries". For instance, if the user selects days 1 and 3, and subsequently clicks on day 3 again, day 1 will remain selected to "update" the range.

Also implemented some fixes on the CSS that became particularly noticeable after this feature.

If there are any problems with this PR or if it will not be accepted for any reason, please let me know so I can take the necessary actions.