Update syn to 2.0, clean up error handling, add closure support to #[with(..)] in derive.

[Gabeperson]

This PR updates syn version to 2.0, adds span reporting for some derive errors, and adds closure support for #[with(..)].

Bevy uses syn 2.0 as well, so once the derive-getters dependency is updated to a version that uses syn 2.0 as well, users won't need to compile both syn 1.0 and 2.0 libraries.

I also changed a few functions to return errors instead of panicking so users get span information for some errors. (I might do another pass in the future to add error messages to all derive errors)

I also added closure support for the #[with(..)] attribute. Example below:

Previously, you'd have to make a separate function for every field you want custom constructors for. This is messy and reduces readability.

#[derive(Default, Bundle, LdtkEntity)]
struct PlayerBundle {
    #[sprite_sheet]
    sprite_sheet: Sprite,
    player: Player,
    #[with(player_rigid_body)]
    rigid_body: RigidBody,
    #[with(player_collider)]
    collider: Collider,
    #[with(player_friction)]
    combine: Friction,
    #[with(player_locked)]
    locked: LockedAxes,
}

fn player_locked(_: &EntityInstance) -> LockedAxes {
    LockedAxes::ROTATION_LOCKED
}

fn player_friction(_: &EntityInstance) -> Friction {
    Friction::new(0.).with_combine_rule(CoefficientCombine::Min)
}

fn player_rigid_body(_: &EntityInstance) -> RigidBody {
    RigidBody::Dynamic
}

fn player_collider(_: &EntityInstance) -> Collider {
    Collider::rectangle(16., 16.)
}

With closure support, this can be shortened:

#[derive(Default, Bundle, LdtkEntity)]
struct PlayerBundle {
    #[sprite_sheet]
    sprite_sheet: Sprite,
    player: Player,
    #[with(|_| RigidBody::Dynamic)]
    rigid_body: RigidBody,
    #[with(|_| Collider::rectangle(16., 16.))]
    collider: Collider,
    #[with(|_| Friction::new(0.).with_combine_rule(CoefficientCombine::Min))]
    combine: Friction,
    #[with(|_| LockedAxes::ROTATION_LOCKED)]
    locked: LockedAxes,
}