MSVC C++20 Support

[Leon0402]

Add Support for Windows MSVC

[The-EDev]

Extra context (I was writing a description for a different PR (same branch though) and @Leon0402 beat me to it 😅)

Crow currently fails to compile on C++20. This is because of the following function:

Crow/include/crow/app.h

Lines 87 to 94 in 6f2eeda

	///Create a route using a rule (**Use CROW_ROUTE instead**)
	template <uint64_t Tag>
	auto route(std::string&& rule)
	-> typename std::result_of<decltype(&Router::new_rule_tagged<Tag>)(Router, std::string&&)>::type
	{
	return router_.new_rule_tagged<Tag>(std::move(rule));
	}

Specifically std::result_of, which was deprecated in C++17 and removed in C++20.
This PR adds a macro to check for C++17 (or later) availability, and based on that replaces std::result_of with std::invoke_result which was introduced in C++17 as a replacement to std::result_of

[luca-schlecker]

LGTM. 🎉 Tested it locally, tests are passing for me and it compiles fine with C++20. 👍 I've tested the helloworld example on Linux.

[The-EDev]

Did the same as @luca-schlecker, seems to be working fine. Will merge once tests are complete