Missing C++ attribute support

[kirillbobyrev]

I'm not sure if it is intended or not, but I wasn't able to find any support for C++ attributes. I would be willing to add them, but I'm not sure which highlighting is intended for the attributes since I couldn't find them anywhere. The following is a list of attributes that could be added to the plugin:

• C++11: noreturn, carries_dependency
• C++ 14: deprecated
• C++17: fallthrough, nodiscard, nodiscard("reason"), maybe_unused
• C++ 20: likely, unlikely, no_unique_address and pre-,post-conditions + assertions: expects and ensures.

[octol]

Thanks for opening the issue so we can keep track of it