Support raw property on returned declarations in visitors

[devongovett]

This enables a raw property to be provided instead of value when returning declarations from JS visitors. This will be parsed as a CSS value for the given property name as if it were written in a CSS file. This should make it easier to return known CSS values rather than constructing the full AST for them manually.

{
  property: 'background',
  raw: 'yellow url(foo.png)'
}

To make this work, we needed an into_owned() method for all values with lifetimes so that we can parse the raw string and store the result in the AST without borrowing from it. This will be useful for other things as well, like substituting variable values. It is derived by a proc macro behind the "into_owned" feature flag.

Another option would have been exposing a parseProperty function that would return a parsed AST from a string, but this would require more back and forth between JS and native which can have some overhead. Allowing raw CSS strings to be returned avoids this.