fix: Make field types invariant for soundness

[dcodeIO]

Discovered in #2537: #2158 introduced unsoundness by making class fields covariant, which is also incompatible with Wasm GC where mutable fields are required to be invariant.

As Wasm GC states:

Structure types support width and depth subtyping

• struct <fieldtype1>* <fieldtype1'>* <: struct <fieldtype2>*
  • iff (<fieldtype1> <: <fieldtype2>)*

Field types are covariant if they are immutable, invariant otherwise

• const <valtype1> <: const <valtype2>
  • iff <valtype1> <: <valtype2>
• var <valtype> <: var <valtype>
• Note: mutable fields are not subtypes of immutable ones, so const really means constant, not read-only

Example of the problem:

class Animal {
  sibling: Animal;
}
class Cat extends Animal {
  sibling: Cat; // unsound
}
class Dog extends Animal {
  sibling: Dog; // unsound
}

(<Animal>new Cat()).sibling = new Dog(); // Cat with Dog sibling is allowed by covariance but incorrect

This PR makes field types invariant (again), but keeps the ability to redeclare fields as long as types are invariant.

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