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/package.json

@@ -67,6 +67,7 @@
     "canvas-roundrect-polyfill": "0.0.1",
     "clsx": "1.1.1",
     "cross-env": "7.0.3",
+    "fractional-indexing": "3.2.0",
     "fuzzy": "0.1.3",
     "image-blob-reduce": "3.0.1",
     "jotai": "1.13.1",