What?

See #76525

Migrate the column block's custom width attribute to use the dimensions.width block support.

Why?

The column block currently manages its own width attribute and a bespoke inspector control, separate from the standard design tooling infrastructure. Moving to block support aligns it with the rest of the system, makes column width available via theme.json and global styles, and removes custom UI code in favor of the existing dimensions panel.

Part of the 7.1 design tooling enhancements tracked in #76525.

How?

• Registers dimensions.width support on the column block with __experimentalSkipSerialization -- the column lives in a flex container, so it needs flex-basis rather than width in the rendered CSS.
• Adds width-to-flex-basis post-processing in WP_Theme_JSON_Gutenberg and transformToStyles (global styles engine) so that theme.json styles resolve correctly for columns.
• Removes the custom width attribute and the ColumnInspectorControls panel from the edit component.
• Adds two deprecation entries to migrate existing content (string width and older numeric width both migrate to style.dimensions.width).
• Updates column width utilities, variations, transforms, and native editor code to read/write from the new location with legacy fallbacks.

Testing Instructions

1. On trunk, create a post with a Columns block. Set various column widths (e.g., 33/66 split, 25/50/25, custom px values). Publish the post.
2. Check out this branch and rebuild.
3. Open the published post on the front end. Verify it renders identically to how it did on trunk.
4. Open the post in the editor. Verify the columns load without block validation errors and the widths display correctly.
5. Save the post. Inspect the block markup and confirm the width is now stored as style.dimensions.width rather than the top-level width attribute.
6. Re-open the post in the editor. Verify it loads cleanly with no validation errors (confirming the re-saved format is the current version).
7. Create a new Columns block and select various width presets (50/50, 33/66, 25/50/25, etc.). Verify widths render correctly in the editor and on the front end.
8. Select an individual column and adjust its width via the dimensions panel in the sidebar. Confirm flex-basis (not width) is applied.
9. Test columns with percentage, px, em, and other unit widths.
10. Insert a Media & Text block, then transform it to Columns. Verify the width split is preserved.
11. Add and remove columns from an existing Columns block. Verify widths recalculate correctly.

Testing Instructions for Keyboard

1. Use Tab/Shift+Tab to navigate into the Columns block and select an individual column.
2. Open the block settings sidebar (Shift+Alt+I or equivalent).
3. Tab to the Width control in the Dimensions section.
4. Change the value using the keyboard and verify the column updates.

Screenshots or screencast