Define `PrettyHelp` class with methods for declaring different parts of help message.
Currently able to produce standard help messages as expected.
Updates chformal to use (only) the new help_v2.
Currently makes use of a global static to track the current help context, allowing register.h to live in blissful ignorance and instead rely on help_v2 implementations calling `auto *help = PrettyHelp::get_current();` and `return true;` to minimise impact on rebuilds (i.e. not requiring every source file to be recompiled).
