feat: add WrenErrorFn callback to WrenConfiguration for custom error reporting

- Add `WrenErrorFn` typedef in wren.h accepting module, line, and message parameters
- Include `errorFn` slot in `WrenConfiguration` struct with documentation comment
- Initialize `errorFn` to NULL in `wrenInitConfiguration()` in wren_vm.c
- Replace direct `fprintf(stderr, ...)` in wren_compiler.c with call to `parser->vm->config.errorFn()`
- Add guard in wren_compiler.c to skip error reporting when `errorFn` is NULL
- Implement `reportError()` in cli/vm.c that writes formatted errors to stderr
- Wire `reportError` into config during `initVM()` in cli/vm.c

src/vm/wren_compiler.c

@@ -394,14 +394,16 @@ static void printError(Parser* parser, int line, const char* label,
   parser->hasError = true;
   if (!parser->printErrors) return;
   
+  // Only report errors if there is a WrenErrorFn to handle them.
+  if (parser->vm->config.errorFn == NULL) return;
+  
   // Format the label and message.
   char message[ERROR_MESSAGE_SIZE];
   int length = sprintf(message, "%s: ", label);
   length += vsprintf(message + length, format, args);
   ASSERT(length < ERROR_MESSAGE_SIZE, "Error should not exceed buffer.");
   
-  fprintf(stderr, "[%s line %d] %s\n", parser->module->name->value, line,
-          message);
+  parser->vm->config.errorFn(parser->module->name->value, line, message);
 }
 
 // Outputs a compile or syntax error. This also marks the compilation as having

src/vm/wren_vm.c

@@ -42,6 +42,7 @@ void wrenInitConfiguration(WrenConfiguration* config)
   config->bindForeignMethodFn = NULL;
   config->bindForeignClassFn = NULL;
   config->writeFn = NULL;
+  config->errorFn = NULL;
   config->initialHeapSize = 1024 * 1024 * 10;
   config->minHeapSize = 1024 * 1024;
   config->heapGrowthPercent = 50;