use `Value` abstraction for `std::variant<Any>`

[aardhyn]

It'd becoming a burden to constantly query and check the underlying types of literal values.

class Value final {}; // could be literal or variable. could be number, string, array, etc.

This type would be intentionally non-generic. throwing runtime type exceptions instead, for the editor to handle.

// literals operate on literals, and return literals...
constexpr bool operator==(const Value& a, const Value& b) const; // evaluates type, checks equality
constexpr Value+(const Literal& a, const Value& b) const;
constexpr Value[]() const;
// etc.

You can create values from primitive types

explicit Value(int value);
explicit Value(bool value);
// etc.

and convert from them

constexpr bool (bool)() const;
constexpr std::string (std::string)() const;

The editor should be responsible for ensuring the right types are used, and valid blocks and expressions are nested together.