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

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

packages/excalidraw/tests/zindex.test.tsx

@@ -46,6 +46,7 @@ const populateElements = (
     height?: number;
     containerId?: string;
     frameId?: ExcalidrawFrameElement["id"];
+    index?: ExcalidrawElement["index"];
   }[],
   appState?: Partial<AppState>,
 ) => {