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

packages/frontend/component/src/utils/index.ts

@@ -1,4 +1,5 @@
 export * from './observe-intersection';
 export * from './observe-resize';
+export * from './shallow-equal';
 export { startScopedViewTransition } from './view-transition';
 export * from './with-unit';

packages/frontend/component/src/utils/shallow-equal.ts

@@ -0,0 +1,43 @@
+// credit: https://github.com/facebook/fbjs/blob/main/packages/fbjs/src/core/shallowEqual.js
+export function shallowEqual(objA: any, objB: any) {
+  if (Object.is(objA, objB)) {
+    return true;
+  }
+
+  if (
+    typeof objA !== 'object' ||
+    objA === null ||
+    typeof objB !== 'object' ||
+    objB === null
+  ) {
+    return false;
+  }
+
+  const keysA = Object.keys(objA);
+  const keysB = Object.keys(objB);
+
+  if (keysA.length !== keysB.length) {
+    return false;
+  }
+
+  // Test for A's keys different from B.
+  for (const key of keysA) {
+    if (
+      !Object.prototype.hasOwnProperty.call(objB, key) ||
+      !Object.is(objA[key], objB[key])
+    ) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+export const shallowUpdater =
+  <T>(newState: T) =>
+  (state: T | null): T => {
+    if (state && shallowEqual(state, newState)) {
+      return state;
+    }
+    return newState;
+  };