two new built in functions

Named `zero` and `unitonly`.

`zero :`
---
It takes a dimensional value (2px, 56%, 7em, ...) as parameter and 
truncates **0unit** to only **0** while rest values remain unaltered.

**Example: zero(0em) // 0  | zero(2px) // 2px | zero(5%) // 5% | zero(0%) // 0**

`unitonly :`
---
It takes a dimensional value (2px, 56%, 7em, ...) as parameter and 
returns the **unit** of the value passed.

**Example: unitonly(2px) // px  | unitonly(2em) // em | unitonly(2%) // %**