Handle whitespace within C++ preprocessor comments

[mattgodbolt]

With reference to: compiler-explorer/compiler-explorer#879, code like:

#if defined(__has_include)
    #if __has_include(<io.h>)
        #include <io.h>
    #endif 
#endif 

would not be coloured correctly. The @whitespace token ate up the leading whitespace before getting to the ^\s* so the ^ wouldn't match. Additionally, there can be whitespace before the include of #include

[mattgodbolt]

Hey @alexdima - just a small ping on this. I'd love to get this in the next release if possible :)

[alexdima]

Thank you!

[mattgodbolt]

Thanks for your tireless work on making Monaco awesome :)