Bring #define (for cource-code-defined text substitution macros) to Vb.Net!

_just porting my old vb.net suggestions from uservoice_

I propose to add a keyword that behaves like C's `#define` to define pre compiler text substitution macros in VB.Net code.

Hopefully this will also allow advanced users to define their own keywords and to unofficially add shortcut for vb' expressive syntax. It would improve productivity because code can be effectively 'compressed', meaning that developers would have to type less to get more!