fix(data-table): allow rowSpan and colSpan on selection column type

[c01dwave]

Description

Fix TypeScript type definition for [TableSelectionColumn] to allow rowSpan and colSpan properties.

Closes #7347

Problem

When using rowSpan or colSpan on a selection column, TypeScript reports an error:
Type '(rowData: T) => number' is not assignable to type 'undefined'.

However, the runtime behavior works correctly - selection cells can be merged as expected.

Root Cause

In commit f56d978 (2021-03-24), when rowSpan and colSpan were added to [TableBaseColumn], they were explicitly set to never on [TableSelectionColumn] This was likely an intentional design decision at the time, but it doesn't match the actual runtime behavior where these properties are supported.

Solution

Change the type definition from:

colSpan?: never
rowSpan?: never

to:

colSpan?: (rowData: T, rowIndex: number) => number
rowSpan?: (rowData: T, rowIndex: number) => number

This aligns the type definition with the actual runtime behavior.