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>

tsconfig.json

@@ -25,6 +25,8 @@
       "@excalidraw/excalidraw/*": ["./packages/excalidraw/*"],
       "@excalidraw/element": ["./packages/element/src/index.ts"],
       "@excalidraw/element/*": ["./packages/element/src/*"],
+      "@excalidraw/fractional-indexing": ["./packages/fractional-indexing/src/index.ts"],
+      "@excalidraw/fractional-indexing/*": ["./packages/fractional-indexing/src/*"],
       "@excalidraw/math": ["./packages/math/src/index.ts"],
       "@excalidraw/math/*": ["./packages/math/src/*"],
       "@excalidraw/utils": ["./packages/utils/src/index.ts"],