feat: #3627823 Optional dividers between rows and columns in the views list components

Issue: https://www.drupal.org/i/3627823

• views-view-unformatted: row_divider and row_divider_color draw the divider atom between rows.
• views-view-grid: column_divider draws a vertical line between columns from md up.

All default off, so existing listings are unchanged. Neither is a Canvas component, so no version hashes move. Used by The Rightup in https://www.drupal.org/i/3627824.

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