Allow parser to support generic data types

[NickGerleman]

Summary:
This diff looks a bit scary, but it's mostly just structural changes of existing code and some deletion, on a well tested path 😅.

The current parser implementation tries to special case "basic" data types. When I was looking at how to add in support for more complex values, such as lists, function notations, and more compounded types, this distinction ends up not making much sense.

Instead of treating some types as basic, this diff instead moves to a model where a user can declare any structure as a CSSDataType, so long as they also supply a parser, which may be visited when iterating through CSS syntax blocks. The user then specifies a list of supported CSS data types to parse, which invokes said parser. E.g.

struct CSSNumber {
  float value{};
};

template <>
struct CSSDataTypeParser<CSSNumber> {
  static constexpr std::optional<CSSNumber> consumePreservedToken(
      const CSSPreservedToken& token) {
    if (token.type() == CSSTokenType::Number) {
      return CSSNumber{token.numericValue()};
    }

    return {};
  }
};

static_assert(CSSDataType<CSSNumber>);

This breaks a whole lot of assumptions I made a year ago, especially around CSSValueVariant which must now be able to handle arbitrary values. For now, for the sake of simplicity, I threw this out, and migrated parser code to use plain-old std::variant, which has a downside of being a bit less optimized in terms of storage. I also ended up completely throwing out CSSDeclaredStyle, since it would majorly need to change, and we're not going to be migrating style storage quite yet. This change also broke the CSSProperties.h property definitions a bit, which we will need for value processing later. I also opted to delete this for now, but will likely copy bits from it for source history later.

Another particular hairy bit, that likely won't bite us in practice, is that some strings may be parseable under different data types. This just adds caller requirement to order the types correctly, instead of precedence being implemented as part of the parser.

Changelog: [Internal]

Differential Revision: D68245734

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This pull request was exported from Phabricator. Differential Revision: D68245734