Hey @iddan, thanks for the great library!

This change enables users to specify highlighted regions of the sheet. My goal is to enable collaborative editing a'la Google Sheets, but it could also be used for user feedback about validation, or something else entirely. I aimed to reuse and somewhat emulate the Selection types and code path. You'll also notice I had to add an initial effect to the Cell component so that highlighting would be possible before the first user interaction. Without that, there isn't bounding rect info available to set the highlighting component coordinates.

Please let me know if there are any changes or additions requested.

Cheers!