Selectable transform issues

[seanforyou23]

We have a selectable transform for react-table, but it isn't clear how it should be used from the selectable table example in our docs.

It's also unused in the TableSelectableDemo in react-integration.

Seems like the canSelectAll api part is integral to the selectable functionality but the relationship between this prop and the transform isn't clear.

There's also a type mismatch mentioned that we'd like to go ahead and address.

Let's fix any issues with the transform, put it to use, and ensure it's well tested/documented.

[stale]

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