- implemented handling for virtual function.

Syntax-wise I chose to make it as strict as possible to reduce the chance of errors: Virtual base functions must be declared with the 'virtual' keyword, and overrides in child classes with the 'override' keyword. This way any mismatch in parameters that otherwise would cause silent failure will outright produce a compile error.

src/scripting/zscript/zcc_parser.h

@@ -33,6 +33,8 @@ enum
 	ZCC_FuncConst		= 1 << 10,
 	ZCC_Abstract		= 1 << 11,
 	ZCC_Extension		= 1 << 12,
+	ZCC_Virtual			= 1 << 13,
+	ZCC_Override		= 1 << 14,
 };
 
 // Function parameter modifiers