Val arithmetic changes

[ickshonpe]

Objective

• A numeric Val with an inner value of 0. should be compatible in operations with other numeric Vals regardless of variant.

  For example, Val::Px(10.).try_add(Val::Percent(0.)) fails with a ValArithmeticError::NonIdenticalVariants error but this operation should succeed since a numeric Val with an inner value of 0. always resolves to 0. regardless of its variant.

• Val arithmetic operations with Auto variants should always fail with a ValArithmeticError::NonEvaluatable error but addition and subtraction of a Val::Auto by a Val::Auto returns Val::Auto.

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Changelog

Val::is_zero:
New method which returns true if the Val is numeric with an inner value of 0..

Val::try_add/try_sub changes:

• If the reciever or argument is Val::Auto returns Err(ValArithmeticError::NonEvaluateable).
• If the receiver or argument is numeric with an inner value of 0. convert it to the variant of the other Val before performing the arithmetic operation.
• Implemented support for the Vw, Vh, VMin and VMax variants.

[alice-i-cecile]

I'd probably prefer a couple more tests, but this change looks good.

Mostly trying to think about what tests we can write that will be more robust than the original code 🤔

[ickshonpe]

We could just make an issue for more tests, it might be an ok non-threatening task for a new contributor to take on etc.

Val's evaluate function needs to be updated to support viewport coordinates as well. I'll add an issue for it.

[bushrat011899]

In #9566, I modify the implementation for PartialEq to explicitly equate zero-values of any unit, which may simplify some of the changes in this PR (e.g., is_zero could be removed and replaced with value == Val::ZERO)

[ickshonpe]

Removed in favour of #9609