feat: fractional indexing (#7359)

* Introducing fractional indices as part of `element.index`

* Ensuring invalid fractional indices are always synchronized with the array order

* Simplifying reconciliation based on the fractional indices

* Moving reconciliation inside the `@excalidraw/excalidraw` package

---------

Co-authored-by: Marcel Mraz <marcel@excalidraw.com>
Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/excalidraw/actions/actionGroup.tsx

@@ -27,6 +27,7 @@ import {
   removeElementsFromFrame,
   replaceAllElementsInFrame,
 } from "../frame";
+import { syncMovedIndices } from "../fractionalIndex";
 
 const allElementsInSameGroup = (elements: readonly ExcalidrawElement[]) => {
   if (elements.length >= 2) {
@@ -140,11 +141,12 @@ export const actionGroup = register({
       .filter(
         (updatedElement) => !isElementInGroup(updatedElement, newGroupId),
       );
-    nextElements = [
+    const reorderedElements = [
       ...elementsBeforeGroup,
       ...elementsInGroup,
       ...elementsAfterGroup,
     ];
+    syncMovedIndices(reorderedElements, arrayToMap(elementsInGroup));
 
     return {
       appState: {
@@ -155,7 +157,7 @@ export const actionGroup = register({
           getNonDeletedElements(nextElements),
         ),
       },
-      elements: nextElements,
+      elements: reorderedElements,
       commitToHistory: true,
     };
   },