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/tests/fixtures/elementFixture.ts

@@ -17,6 +17,7 @@ const elementBase: Omit<ExcalidrawElement, "type"> = {
   groupIds: [],
   frameId: null,
   roundness: null,
+  index: null,
   seed: 1041657908,
   version: 120,
   versionNonce: 1188004276,