fix: Fractional index validation (#11258)

- Vendored fractional-indexing and converted to TypeScript
- Stricter index format validation in fractional-indexing
- Added format validation to fractional index validation

---
Signed-off-by: Mark Tolmacs <mark@lazycat.hu>
Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/element/src/fractionalIndex.ts

@@ -1,7 +1,10 @@
-import { generateNKeysBetween } from "fractional-indexing";
-
 import { arrayToMap } from "@excalidraw/common";
 
+import {
+  validateOrderKey,
+  generateNKeysBetween,
+} from "@excalidraw/fractional-indexing";
+
 import { mutateElement, newElementWith } from "./mutateElement";
 import { getBoundTextElement } from "./textElement";
 import { hasBoundTextElement } from "./typeChecks";
@@ -382,6 +385,13 @@ const isValidFractionalIndex = (
     return false;
   }
 
+  try {
+    // Format validation
+    validateOrderKey(index);
+  } catch {
+    return false;
+  }
+
   if (predecessor && successor) {
     return predecessor < index && index < successor;
   }