Validate global imports (#8487)

* Add a RuntimeGlobal class to wrap the global's runtime value as well
as its definition so we can check its type + mutability for import
validation
* Global imports are invariant for mutable globals and covariant for
immutable globals:
<img width="841" height="428" alt="image"
src="https://github.com/user-attachments/assets/748cb719-6de9-4dfd-9719-e4051dc31c45"
/>. Mutability must match as well.
* Fix old_import.wast to account for instantiations which should
correctly fail. This test is now mostly the same as
test/spec/imports.wast, but there are small differences so I plan to
keep it until the upstream testsuite imports.wast passes.


Example error message:

```
[trap Imported global Mref_ex.g-var-func with type: (mut (ref func)) isn't compatible with import declaration: (import "Mref_ex" "g-var-func" (global $gimport$0 (mut funcref)))
]
```

Part of #8261.