calc easier to extend syntax

[RobertasJ]

A pr where instead of using symbols for custom values, we have variables instead, list of variables:

• root
• root.cross
• root.width
• root.height
• parent
• parent.cross
• parent.width
• parent.height
• scale
  in addition to that there are now new functions one can use within calc:
• min(value1, value2, value3, ...)
• max(value1, value2, value3, ...)
• clamp(minimum_value, value, maximum_value)

an always square rect element example:

rsx! {
    rect {
        width: "100", // imagine this value changing arbitrarily
        height: "400", // imagine this value changing arbitrarily
        rect {
            width: "calc(min(parent, parent.cross))", 
            height: "calc(min(parent, parent.cross))"
        }
    }
}

i marked it as a draft as im sure this is pretty new. might be interesting to discuss if this syntax would be good or not.