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/cli/vm.c

@@ -156,6 +156,11 @@ static void write(WrenVM* vm, const char* text)
   fflush(stdout);
 }
 
+static void reportError(const char* module, int line, const char* message)
+{
+  fprintf(stderr, "[%s line %d] %s\n", module, line, message);
+}
+
 static void initVM()
 {
   WrenConfiguration config;
@@ -165,6 +170,7 @@ static void initVM()
   config.bindForeignClassFn = bindForeignClass;
   config.loadModuleFn = readModule;
   config.writeFn = write;
+  config.errorFn = reportError;
 
   // Since we're running in a standalone process, be generous with memory.
   config.initialHeapSize = 1024 * 1024 * 100;