Change CSSValue types to have immutable value semantics

[Sam Weinig]

To create a clear boundary and allow for more caching opportunities, we should convert the CSSValue types to have immutable value semantics, moving whatever remains to the style / style building system.

In the end, I would like to have a type for each possible top level css value, and make CSSValue itself a type-erasing wrapper of sorts:

struct CSSValue {
    std::variant<
        // Each css value type here.
    > value;
};


We can use this opportunity to create more strongly typed results coming out of the parser, so style building does not have to do quite as much type checking or assuming.

[Radar WebKit Bug Importer]

<rdar://problem/101479589>