Still not working, but a base skeleton is available at least

Co-authored-by: Ghaith Hachem <[email protected]>

A very basic example as seen below will now work
```
VAR_GLOBAL
    globalVar : DINT := 10;
END_VAR

TYPE Parent:
    STRUCT
        foo : REF_TO DINT;
    END_STRUCT
END_TYPE

TYPE Child:
    STRUCT
        instance : Parent := (foo := REF(globalVar));
    END_STRUCT
END_TYPE

FUNCTION main
    VAR
        localChild : Child;
    END_VAR

    printf('Value: %d$N', localChild.instance.foo^);
END_FUNCTION
```
For that to work the `indext_struct_type` function needed to add the context defining which struct it currently operates in for the const evaluator to copy over into the `InitData` struct.

Also the resolver needed some extra logic with regards to the init `self` argument, though not entirely sure if needed (specifically the `resolve_reference_expression` needs to match on a `ReferenceExpr`?)

Finally there still seems to be some issues regarding the const evaluator, since one validation ("Some initial values were not generated")  reported an error.

Still, a first prototype 🎉

Something like `var : StructA := (instanceB := (instanceC := (value :=
5)))` would previously not work because the resolver had incorrect
metadata when recursively descending into these assignments.
Specifically it always assumed the name of the left-hand side to be
whatever the first element was when visiting the right-hand side as a
whole, i.e. it assumed `StructA` when in reality it needed to be
`StructA` for `(instanceB := (...))`, `StructB` for `instanceC := (...)`
and `StructC` for `value := 5`.