Accessibility for sortable table headers in slate table

[kreafox]

Description:
This PR enhances the accessibility of sortable slate table headers, ensuring they are keyboard navigable.

Steps to Reproduce:

1. Create a table block on any page and make it sortable
2. Try to sort the table using the keyboard (by pressing Enter or Space on the sortable header cells)

Expected Behavior:
The sortable table headers should be focusable and allow sorting when activated with the Enter or Space key, enabling keyboard navigation.

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