Enable multi-line sheet cells

Some users want to be able to hit alt-enter in order to start a new line in a cell.

I believe we should be able to enable this using `wordWrap: true`:

https://bossanova.uk/jspreadsheet/v2/examples/text-wrapping

Will need to determine if we want to enable that for all cells, or have a way for users to toggle it.

No repository file or test is named. Start with the linked text-wrapping example and the current cell-editing behavior, then determine whether wordWrap: true should apply to every cell or be user-configurable; done means Alt-Enter creates a new line and the chosen scope is documented and covered by verification.

Written by the indexing model from the issue text.