Major refactoring of the #[var] and #[export] inner workings, in particular regarding handling of property hints, hint strings and usage flags.

Before, property customization were widely scattered across the whole codebase, with quite some redundancy and non-intuitive data flows. For example, typed and untyped arrays need to be registered completely different, then there are subtle differences regarding #[var] and #[export] contexts, plus other nuances like VariantType::NIL being used as both Variant and void.

This PR brings an end to this mess with a central enum, being returned from GodotConvert::godot_shape():

#[non_exhaustive]
pub enum GodotShape {
    Variant,
    Builtin {
        variant_type: VariantType,
    },
    Class {
        class_id: ClassId,
        heritage: ClassHeritage,
    },
    TypedArray {
        element_shape: Box<GodotShape>,
    },
    TypedDictionary {
        key_shape: Box<GodotShape>,
        value_shape: Box<GodotShape>,
    },
    Enum {
        variant_type: VariantType,
        enumerators: Cow<'static, [Enumerator]>,
        godot_name: Option<CowStr>,
        is_bitfield: bool,
    },
    Custom {
        variant_type: VariantType,
        var_hint: PropertyHintInfo,
        export_hint: PropertyHintInfo,
        class_name: Option<CowStr>,
        usage_flags: PropertyUsageFlags,
    },
}

Removes the following APIs:

• Var::var_hint()
• Export::export_hint()
• GodotType::property_info()
• GodotType::property_hint_info()
• Tons of tiny helpers, like PropertyHintInfo methods: var_array_element<T>, export_array_element<T>, var_dictionary_element<K, V>, export_dictionary_element<K, V>, export_packed_array_element<T>, export_gd<T>, export_dyn_gd<T, D>, ...

Furthermore, this opens up the Element trait to allow enums to be stored in arrays and dictionaries. Adds extensive tests for enums, bitfields, and nested ones in array/dictionary.

Closes #353.
Paves the way for a future #890.
Diff is +700, however this also adds a new feature (enum support). Plus, 260 of it are new tests, and 210 are extra docs.