feat: Add enums to the HybridObject

[mrousavy]

Adds support for enums to HybridObject:

When declaring custom enums there's two steps you need to do:

1. Create your enum (e.g. in TestEnum.h)

enum TestEnum {
    FIRST,
    SECOND,
    THIRD
};

2. Add the enum to the EnumMapper by editing cpp/jsi/EnumMapper.h:

template<>
struct EnumMapper<TestEnum> {
public:
    static constexpr TestEnum fromJSUnion(const std::string& jsUnion) {
        if (jsUnion == "first") return FIRST;
        if (jsUnion == "second") return SECOND;
        if (jsUnion == "third") return THIRD;
        throw invalidUnion(jsUnion);
    }
    static std::string toJSUnion(TestEnum value) {
        switch (value) {
            case FIRST: return "first";
            case SECOND: return "second";
            case THIRD: return "third";
        }
        throw invalidEnum(value);
    }
};

This way compile-time safety is guaranteed.

All other approaches to parse enums from C++ to JS are not compile-time but rather run-time, so currently there's no way around editing EnumMapper.h unless we do runtime type checking.