[css-text-decor] Add longhands for `text-shadow`

[SebastianZ]

As discussed in #4431, the text-shadow property should be turned into a shorthand like it was done with box-shadow.

Taking the proposed syntax from the other issue and the one used for box-shadow, the longhands may look like this:

text-shadow-offset: none | <length>{2}#
text-shadow-color: <color>#
text-shadow-blur: <length [0,∞]>#
text-shadow-spread: <length>#
text-shadow: <single-text-shadow>#
<single-text-shadow>: <'text-shadow-color'>? && \
          [ <'text-shadow-offset'> [ <'text-shadow-blur'> <'text-shadow-spread'>? ]? ]

Sebastian

[cdoublev]

Aside: it would be nice to explicitly define text-shadow-offset as the coordinating list base property; this terminology is missing for box-shadow.