[improvement] Support Injected Alias Types for Packages

[montymxb]

According to the 6.1 spec, these pair of statements are added to the package-level of every compilation:

Define alias __SIGNED_INT signed fixed bin(p1,0);

Define alias __UNSIGNED_INT unsigned fixed bin(p2,0);

A standalone example shows a simple usage here:

 pack: package;
 p: proc options(main);
    dcl si type __signed_int;
    dcl ui type __unsigned_int;
 end p;
 end pack;

Although not part of a physical library, we should be able to link these to a built-in (or equivalent) with the expected type alias definitions.