[todo]: closure or Indexing, etc. should follow the scope of where they are declared.

[sunrabbit123]

interface A {
    a: 'a';
}
namespace C {
  interface B1 extends A {
      b: 'b';
      c: OldDiff<keyof this, keyof A>;
  }
}

type c1 = C.B1['c']; // 'c' | 'b'

Currently, our STC does not have a scope for where types are declared.
This requires quite a bit of structure swapping.

related issue

https://github.com/dudykr/stc/pull/1059/files#r1288473639