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/include/wren.h

@@ -58,6 +58,9 @@ typedef WrenForeignMethodFn (*WrenBindForeignMethodFn)(WrenVM* vm,
 // Displays a string of text to the user.
 typedef void (*WrenWriteFn)(WrenVM* vm, const char* text);
 
+// Reports an error to the user.
+typedef void (*WrenErrorFn)(const char* module, int line, const char* message);
+
 typedef struct
 {
   // The callback invoked when the foreign object is created.
@@ -126,6 +129,13 @@ typedef struct
   //
   // If this is `NULL`, Wren discards any printed text.
   WrenWriteFn writeFn;
+  
+  // The callback Wren uses to report errors.
+  //
+  // When an error occurs, this will be called with the module name, line
+  // number, and an error message. If this is `NULL`, Wren doesn't report any
+  // errors.
+  WrenErrorFn errorFn;
 
   // The number of bytes Wren will allocate before triggering the first garbage
   // collection.