Bug 249351

During reification of CSSValues, if encountering a CSSValueList, we would:
1. Return the first value in the list for StylePropertyMap.get()
2. Return each item in the list in an array for StylePropertyMap.getAll()

However, we're only supposed to do this if he property is a list-valued property [1][2]. For non-list valued properties, we're should:
1. Return a generic CSSStyleValue wrapping the list for StylePropertyMap.get()
2. Return an array containing a single item that is a generic CSSStyleValue wrapping the list, for StylePropertyMap.getAll()

See:
[1] https://drafts.css-houdini.org/css-typed-om/#list-valued-properties
[2] https://drafts.css-houdini.org/css-typed-om/#reify-stylevalue