feat: add user-defined state data pointer and accessors to WrenVM

Add a `userData` void pointer field to `WrenConfiguration` and expose `wrenGetUserData`/`wrenSetUserData` functions for associating arbitrary state with a VM instance. Also update the `WrenErrorFn` signature to include the `WrenVM*` parameter, and propagate the VM pointer through all error callback invocations in the compiler, debug, and runtime paths.

src/include/wren.h

@@ -96,7 +96,7 @@ typedef enum
 // Each of those has the module and line where the method or function is
 // defined and [message] is the name of the method or function.
 typedef void (*WrenErrorFn)(
-    WrenErrorType type, const char* module, int line, const char* message);
+    WrenErrorType type, const char* module, int line, const char* message, WrenVM* vm);
 
 typedef struct
 {
@@ -206,6 +206,10 @@ typedef struct
   //
   // If zero, defaults to 50.
   int heapGrowthPercent;
+
+  // User defined state data associated with the WrenVM.
+  void* userData;
+
 } WrenConfiguration;
 
 typedef enum
@@ -447,4 +451,10 @@ void wrenGetVariable(WrenVM* vm, const char* module, const char* name,
 // runtime error object.
 void wrenAbortFiber(WrenVM* vm, int slot);
 
+// Returns the user data associated with the WrenVM.
+void* wrenGetUserData(WrenVM* vm);
+
+// Sets user data associated with the WrenVM.
+void wrenSetUserData(WrenVM* vm, void* userData);
+
 #endif