DataTable: Add labels to convey how to resize columns

[interactivellama]

Currently resizing columns with resizable is not fully accessible. The keyboard can be used, but there are no directions in how to use it.

Current proposal is to use the feedback/label/UX of the slider ARIA widget and report the current pixel number to the user.

Currently resizing is implicit to the table header cell when in Salesforce's custom "edit mode"

[stale[bot]]

This issue has been automatically marked as stale, because it has not had recent activity. It will be closed if no further activity occurs. Maintainers are responsible for tech debt and project health. This is most likely a new components or component feature request. Please submit a pull request for or request feedback on this feature. Thank you.