feat: refactor mixin "xy-cell-offset()" and add corresponding function

feat: refactor mixin "xy-cell-offset()" and add corresponding function

Changes:
- Add function `xy-cell-offset()`: Returns the appropriate CSS value to offset a cell.
- Refactor mixin `xy-cell-offset()` to use its corresponding function.

**BREAKING CHANGE**: The `$breakpoint` argument in the `xy-cell-offset()` mixin does not support raw value anymore. Use `$gutters` instead.