Allow multiples of `.25` in `aspect-*` fractions (#19688)

This PR reduces the restrictions of the `aspect-*` utilities when
dealing with fractional values.

Up until now, the numbers had to be positive integers, so `aspect-1/2`
was valid, but `aspect-8.5/11` was not.

This PR allows for any multiple of `.25` as a valid value, so
`aspect-8.5/11` is now valid, but `aspect-8.3/11` is not, this will
still require `aspect-[8.3/11]` arbitrary value syntax to be valid.

This behavior of allowing multiples of `.25` is consistent with other
utilities that handle bare values such as `w-2.5`.

## Test plan

1. Existing tests pass
2. Added a test for `aspect-8.5/11`

Fixes: #19663
Closes: #19680, #19669