Use _shared as the CMDB shared configuration

[roleyfoley]

Expected Behaviour

I can use the folder name _shared when I want to provide shared files that are included as part of the layered CMDB file system

Current Behaviour

I use a folder called shared when I want to provide shared files

Possible Solution

Add support for either using _shared or shared for shared files. Raise an error if both are used to avoid things getting messy

Context

This would align with our use of the _ value within the CMDB configuration to highlight that this is a special function or one provided by the engine.