modules: Add a module for DSL introspection

[dcbaker]

This module provides methods for looking at the types of variables. The main thought for this is values returned from user data, such as meson.get_external_property(), and allowing choices to be made based on type information.

This implementation is rather incomplete, and there is no documentation provided (yet?).

Largely at this point I want to solicit feedback from the community, as I am concerned that this may help some niche cases, but be as a whole be a determent.

Cases that have come up where it would be helpful:

1. meson.get_external_property(), where the type cannot be known

My hope is that we can either build consensus to move forward with this OR to build consensus to add language to the documentation as to why we don't have this, and don't want to add it, similar to the section on functions and macros.

[bruchar1]

Is there other cases where the type is unknown? I fear that having such a module would be more confusing than helpful...

What I would rather propose would be to return a property object, with converter methods (to_array(), to_int(), to_string()) and, if needed, introspection methods (is_array(), is_string(), etc.)