Support dynamic return type for `WP_Syntex\Polylang\Options\Abstract_Option::get()`.

[Hug0-Drelon]

What?

As already done for WP_Syntex\Polylang\Options\Options::get() in OptionsGetDynamicMethodReturnTypeExtension, it would be nice to support dynamic return type for inner option object (WP_Syntex\Polylang\Options\Abstract_Option).