feat(core): drop doc onto split view (#9487)

fix AF-2068, AF-2069, AF-1175, AF-2061, AF-2079, AF-2034, AF-2080, AF-1960, AF-2081

1. replace `dnd-kit` with `@atlaskit/pragmatic-drag-and-drop`
2. allow creating split views by drag & drop the following
   a. WorkbenchLinks (route links), like journals, trash, all docs
   b. doc refs
   c. tags/collection
3. style adjustments to split view
4. remove split view's feature flag and make it GA for electron

https://github.com/user-attachments/assets/6a3e4a25-faa2-4215-8eb0-983f44db6e8c

tests/kit/src/utils/app-tabs.ts

@@ -0,0 +1,52 @@
+import type { Page } from '@playwright/test';
+import { expect } from '@playwright/test';
+
+export async function expectActiveTab(
+  page: Page,
+  index: number,
+  activeViewIndex = 0
+) {
+  await expect(
+    page
+      .getByTestId('workbench-tab')
+      .nth(index)
+      .getByTestId('split-view-label')
+      .nth(activeViewIndex)
+  ).toHaveAttribute('data-active', 'true');
+}
+
+export async function expectTabTitle(
+  page: Page,
+  index: number,
+  title: string | string[]
+) {
+  if (typeof title === 'string') {
+    await expect(page.getByTestId('workbench-tab').nth(index)).toContainText(
+      title
+    );
+  } else {
+    for (let i = 0; i < title.length; i++) {
+      await expect(
+        page
+          .getByTestId('workbench-tab')
+          .nth(index)
+          .getByTestId('split-view-label')
+          .nth(i)
+      ).toContainText(title[i]);
+    }
+  }
+}
+
+export async function expectTabCount(page: Page, count: number) {
+  await expect(page.getByTestId('workbench-tab')).toHaveCount(count);
+}
+
+export async function closeTab(page: Page, index: number) {
+  await page.getByTestId('workbench-tab').nth(index).hover();
+
+  await page
+    .getByTestId('workbench-tab')
+    .nth(index)
+    .getByTestId('close-tab-button')
+    .click();
+}