[css-color-5] Add blurple as an official CSS color

[magnus195]

We got a new color circulating the internet. It's a color everyone has seen but which doesn't work on CSS. I'm talking about blurple. I was trying to make a "Sign in with Discord" button and was shocked that it didn't exist.

It is the main color of Discord, but also pretty nice to look at. That's why I'm voting for adding it.
The color code for regular blurple is #7289DA or rgb(114, 137, 218). Dark blurple is #4E5D94 or rgb(78, 93, 148).

[astearns]

We are not planning on adding any more named colors to CSS

https://wiki.csswg.org/faq#adding-more-named-colors