feat: onRowAppended and onColumnAppended

[BrianHung]

• adds an onColumnAppended function
• allows for onRowAppended to be called without trailingRow when new selection would move it beyond row limit

[Copilot]

Pull Request Overview

Adds support for column-appending alongside existing row-appending in both the core component and test harness, and updates API exports and selection behavior for trailing rows.

• Introduce onColumnAppended callback and appendColumn method in DataEditor.
• Extend test utilities (EventedDataEditor) to simulate appended columns and add corresponding tests.
• Export emptyGridSelection and adjust trailing-row logic to be driven by trailingRowOptions.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/core/test/test-utils.tsx	Added extraCols state, onColumnAppended hook, and updated props in EventedDataEditor
packages/core/test/data-editor.test.tsx	New tests for appendRow without a trailing row and for appendColumn via ref
packages/core/src/index.ts	Exported emptyGridSelection
packages/core/src/data-editor/data-editor.tsx	Added onColumnAppended prop, implemented appendColumn, and updated selection/movement logic

Comments suppressed due to low confidence (2)

packages/core/test/test-utils.tsx:280

• [nitpick] The variable name c is ambiguous; consider renaming it to col for clarity.

            const [c] = cell;

packages/core/src/data-editor/data-editor.tsx:709

• The JSDoc for appendColumn documents row but omits the openOverlay parameter. Add @param openOverlay to keep the documentation in sync with the signature.

     * @param row The row index to focus in the new column.

[lukasmasuch]

I was able to test the new onRowAppended behaviour without a trailing row. But how does it work with onColumnAppended? Using enter will always go down a row, right?

[BrianHung]

I was able to test the new onRowAppended behaviour without a trailing row. But how does it work with onColumnAppended? Using enter will always go down a row, right?

Tab works to go right; shift-tab to go left.

[lukasmasuch]

LGTM 👍