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/element/newElement.ts

@@ -55,6 +55,7 @@ export type ElementConstructorOpts = MarkOptional<
   | "angle"
   | "groupIds"
   | "frameId"
+  | "index"
   | "boundElements"
   | "seed"
   | "version"
@@ -89,6 +90,7 @@ const _newElementBase = <T extends ExcalidrawElement>(
     angle = 0,
     groupIds = [],
     frameId = null,
+    index = null,
     roundness = null,
     boundElements = null,
     link = null,
@@ -114,6 +116,7 @@ const _newElementBase = <T extends ExcalidrawElement>(
     opacity,
     groupIds,
     frameId,
+    index,
     roundness,
     seed: rest.seed ?? randomInteger(),
     version: rest.version || 1,