Ambiguous AST node: implement resolve to variables

For more information on what is the "ambiguous AST node", [see this discussion](https://github.com/LanguageDev/Yoakke/discussions/139#discussioncomment-2238239).

Currently, it only resolves to a declaration but not a variable definition. For example, this seems to be broken:
```c
typedef int xxx;
xxx(foo); // declares a variable foo of type xxx, the same as "xxx foo;"
```

We should support this construct as well as the function call.

Look for the number `213` in the code to find clues to implement this feature.