Allow showing a validation message with `validateCell`

Input validation via `validateCell` is quite useful. But it can also be confusing for the user since there is no native way of showing why the input is invalid. I think it would currently require implementing a dedicated cell overlay editor to show such a message.

Maybe we can extend some of the native cell overlay editors to support showing a message explaining why an input is invalid.

Proposed API:

```
readonly validateCell?: (cell: Item, newValue: EditableGridCell) => string | boolean | EditableGridCell;
```

If a string is returned, it is interpreted as a validation message explaining the constraint. Thereby, the overlay editor needs to somehow show this to the user. Maybe it is possible to use the [`setCustomValidity`](https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation#constraint_combining_several_fields_postal_code_validation) functionality of native HTML input elements for this. E.g. see [this example](https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation#constraint_combining_several_fields_postal_code_validation):