Create the AST based on the parser output

[FlorianChevassu]

The "AST" exposed by the parser follows the CMake documentation, and do not expose all the information we need.
The final AST should:

• Mirror the parser AST, with cleaned "arguments": Arguments represent a pack of arguments (Arguments enclosed in ()), and Argument is a single argument.
• Add AST nodes for the arguments structure: StringArgument, VariableArgument, etc.