Various places in the spec allow for `<number> | <percentage>`, but this
is either/or, and they are not allowed to be combined like dimensions
and percentages are. (For example, `calc(12 + 50%)` is never valid.)
User code generally doesn't need to care about this distinction, but it
does now need to check if a calculation resolves to a number, or to a
percentage, instead of a single call.
The existing parse_number_percentage[_value]() methods have been kept
for simplicity, but updated to check for number/percentage separately.
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