Feature: Checkboxes

[dg-ac]

To ensure compatibility with Excel, we could start with a simple implementation.
Checkboxes can be added by selecting a cell and clicking the respective button in the toolbox.

Once clicked, checkboxes will have 2 states: TRUE (checked) and FALSE (unchecked). Cell references to these cell should take that into account.

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Notes:

1. Checkboxes can be deleted by hitting Del
2. Cells that include checkboxes cannot be overwritten
3. We’re using the icons square-check and check, from Lucide

[nhatcher]

This is how Google sheets does it:

and

It is incompatible with Excel

[dg-ac]

@nhatcher Please take a look at the spec. I think that in the first iteration we could keep it simple. If we do that, we won't break compatibility with Excel, right?