[css-backgrounds-3][css-text-decor-4] Move `<shadow>` to CSS Text Decoration 4

[cdoublev]

All CSS Backgrounds 3 definitions have recently been incorporated into CSS Backgrounds 4 or CSS Borders 4, except <shadow>, if I am not mistaken.

Tools like w3c/webref could simplify their logic if it was not the only remaining definition.

<shadow> is replaced with <spread-shadow> in CSS Borders 4, but is still used for text-shadow in CSS Text Decoration 4. I suspect that using <spread-shadow> for text-shadow instead would be fine though.

I admit I do not know if the definitions of CSS Backgrounds 4 and CSS Borders 4 are intended to be backported to CSS Backgrounds 3 in the future, or if CSS Borders 3 will be created, etc.

Depending on that, renaming <spread-shadow> to <shadow> could be another option.

@SebastianZ @tidoust