Add values-4.vim

This is a syntax file for CSS Values and Units Module Level 4.
See also: https://www.w3.org/TR/2018/CR-css-values-3-20180814/

after/syntax/css/values-4.vim

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+syn match cssValueLength contained "[-+]\=\d\+\(\.\d*\)\=\(vi\|vb\|ic\|cap\|lh\|rlh\)"
+syn region cssFunction contained matchgroup=cssFunctionName start="\<\(min\|max\|clamp\|toggle\)\s*(" end=")" oneline keepend