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>

excalidraw-app/vite.config.mts

@@ -75,6 +75,13 @@ export default defineConfig(({ mode }) => {
           find: /^@excalidraw\/utils\/(.*?)/,
           replacement: path.resolve(__dirname, "../packages/utils/src/$1"),
         },
+        {
+          find: /^@excalidraw\/fractional-indexing$/,
+          replacement: path.resolve(
+            __dirname,
+            "../packages/fractional-indexing/src/index.ts",
+          ),
+        },
       ],
     },
     build: {