Allow percentages in fill-opacity, stroke-opacity, & stop-opacity, to match CSS WG changes

[AmeliaBR]

The CSS WG has resolved to allow percentages interchangeable with decimals in the opacity property and in all color functions that take an alpha value. I don't have the details yet, but I'm sure this was mostly to match with author expectations. opacity: 50% is just easier for many people to grok, compared to opacity: 0.5.

It would therefore expect that, to keep author expectations met, we would want to update fill-opacity, stroke-opacity, and stop-opacity to also allow <number>|<percentage>, with percentages relative to 1.

[AmeliaBR]

Discussion from the CSS WG transcripts, pending official minutes.

[tabatkins]

👍

(The change was actually well over a year old; I made it when I first started editting Color 4. We might have even resolved on it back then. But hey, nothing wrong with repeating the repetition.)