Add `String::New` for `std::string_view`

[cacharle]

It would be nice to have a string "constructor" which takes a string_view

Could be as simple as:

Napi::String::New(napi_env env, std::string_view value)
{
    return Napi::String::New(env, value.data(), value.size());
}

[legendecas]

@cacharle thanks for raising the issue! Would you like to file a PR for this?

Additionally, we could also review other places that could take an std::string_view, where it takes a string pointer.