Replace deprecated substr() with slice() in the codebase

### Summary
The `substr()` method is deprecated in JavaScript and may cause issues or warnings in future versions. To ensure better compatibility and prevent potential runtime issues, we should replace all occurrences of `substr()` with `slice()`.

### Details
Currently, the following functions in `index.js` file contain instances of `substr()`:
- JSONCookie(str) {}
- signedCookie (str, secret) {}

These should be refactored to use `slice()` instead.

### Why It Should Be Fixed
JavaScript's `substr()` method has been deprecated. Refactoring to use `slice()` will future-proof the code and prevent potential issues in modern JavaScript environments. This change is necessary for compatibility with new ECMAScript standards and best practices.

### Suggested Solution
Replace `substr()` with `slice()` in all occurrences.

### References
For more details on the deprecation of `substr()`, see the MDN documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr