Fix line break behavior in rich-text example

Related to ianstormtaylor#5671

Update `insertSoftBreak` to insert a line-break instead of splitting nodes

* Modify `packages/slate/src/editor/insert-soft-break.ts` to use `Transforms.insertText` to insert a line-break character instead of splitting nodes


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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/ianstormtaylor/slate/issues/5671?shareId=fcda97f9-03f1-49f6-8cbc-4fc9f4c95470).

packages/slate/src/editor/insert-soft-break.ts

@@ -2,5 +2,5 @@ import { Transforms } from '../interfaces/transforms'
 import { EditorInterface } from '../interfaces/editor'
 
 export const insertSoftBreak: EditorInterface['insertSoftBreak'] = editor => {
- Transforms.splitNodes(editor, { always: true })
+ Transforms.insertText(editor, '\n')
 }