Use regexp syntax for "string"

Co-authored-by: Caleb Hearon <[email protected]>

lib/parse-font.js

@@ -9,7 +9,7 @@ const styles = 'italic|oblique'
 const variants = 'small-caps'
 const stretches = 'ultra-condensed|extra-condensed|condensed|semi-condensed|semi-expanded|expanded|extra-expanded|ultra-expanded'
 const units = 'px|pt|pc|in|cm|mm|%|em|ex|ch|rem|q'
-const string = `'((\\\\'|[^'])+)'|"((\\\\"|[^"])+)"|[\\w\\s-]+`
+const string = /'((\\'|[^'])+)'|"((\\"|[^"])+)"|[\w\s-]+/.source
 
 // [ [ <‘font-style’> || <font-variant-css21> || <‘font-weight’> || <‘font-stretch’> ]?
 //    <‘font-size’> [ / <‘line-height’> ]? <‘font-family’> ]