Add init_attr<T>

[saki7]

init_attr<T> is a special attr parser that resets the variable and always succeeds.

This can be used for constructing constexpr instance of a parser even when T has dynamically allocated storage. For example, normal attr(std::vector<int>{}) cannot be assigned to a constexpr instance, but init_attr<std::vector<int>> can.

[cppwarningnotifier]

Environment				C++23	C++26
x4	Clang	21	Debug	✅success	✅success
			Release	✅success	✅success
	GCC	14	Debug	✅success	✅success
			Release	✅success	✅success
	MSVC	2022	Debug	✅success	✅success
			Release	✅success	✅success
		2026	Debug	✅success	✅success
			Release	✅success	✅success