std::string in an extra-properties.yaml

[brandonkimbk]

Even though std::string is part of the supported types of Value std::variant, it's deceivingly hard to use.

I was running into this issue: https://stackoverflow.com/questions/44086269/why-does-my-variant-convert-a-stdstring-to-a-bool where:
"The type of "hello" is const char [6], which decays to const char *. The conversion from const char * to bool is a built-in conversion, while the conversion from const char * to std::string is a user-defined conversion, which means the former is performed."

The fix was to declare the string using a Folded Block Scalar > and then wrapping the string in std::string("hello") for example.

A comment / documentation somewhere of this behavior may be useful as this is not very obvious.