Change the data structure to store selections in the selection store

[rajatvijay]

Problem

The current structure is only capable to store the selected cells.
selectedCells: WritableSet<string>;
This is not usable when the table has no rows hence no cells but the user should still be able to select columns and edit their properties as mentioned in #1684

The rationale behind current implementation

The current implementation is based on the fact that when a cell is selected, a row and a column are also selected by default. Row selection can be described as selecting all the cells in a particular row.
Column selection can be described as selecting all the cells in a particular column.

Proposed solution

@pavish proposed the following design:

The selection store to utilize a different structure instead of just the identifier string array, something like:

{
   type: 'cell' | 'column' | 'row',
   identifiers: string[],
}

The identifiers here would depend on the type of selection.

[seancolsen]

I just merged #1733 which moves us forward with the "current implementation", but I do think it's worth considering ways to improve this code.

Although I initially expressed support for the current implementation, I think @pavish has a good idea (if I'm understanding it correctly). To flesh this idea out a bit more I'd propose the following:

In src/stores/table-data/selection.ts...

1. Rename the Selection class to SelectionStore.

2. Use the following:

   interface SelectionOfCells {
     basis: 'cells';
     cellIds: ImmutableSet<string>;
   }
   interface SelectionOfColumns {
     basis: 'columns';
     columnIds: ImmutableSet<number>;
   }
   
   type Selection = SelectionOfCells | SelectionOfColumns;
   
   class SelectionStore {
     // ...
   
     selection: Writable<Selection>;
   
     // ...
   }

This way, the selection is either a selection of cells or a selection of columns, but never both. The SelectionOfColumns type would only be used when the table is empty, similar to how we're currently doing it. I don't currently see a need for tracking a selection of rows, but if such a need arose in the future, we could modify the Selection type accordingly.