Add a check in the `field()` function to detect when a field is referenced outside of a class context. When `compiler->fields` is NULL (indicating no enclosing class), emit a clear error message "Cannot reference a field outside of a class definition." and assign a dummy field index of 255 to allow continued parsing with minimal cascading errors.
Also includes minor cleanup: remove two stale TODO comments about EOF checking and better error messages in `nextToken()` and `consume()`, and add a new test file `test/field/outside_class.wren` that verifies the error is produced for top-level field assignment.
Replace the fixed-size `MAX_PINNED` array of `Obj*` pointers with a dynamically allocated linked list of `PinnedObj` nodes, removing the 16-object limit and eliminating the `numPinned` counter. Update `pinObj` to accept a caller-provided `PinnedObj` stack variable and link it at the head of the list, and simplify `unpinObj` to pop the head without requiring the object argument. Adjust `wrenNewVM` to initialize the new `pinned` pointer to NULL, and modify `collectGarbage` to iterate the linked list instead of the array.
