feat: add configurable write callback for System.print output delegation

Add a `WrenWriteFn` callback to `WrenConfiguration` that allows host applications to control how text from `System.print()` and related functions is displayed. When set, the VM invokes this callback instead of directly calling `printf`. If the callback is NULL, printed text is silently discarded. The CLI's `initVM()` now registers a simple `write` function that prints to stdout, while the core `system_writeString` primitive checks for the callback before outputting. This change also refactors `wrenInitConfiguration` to set `defaultReallocate` as the default reallocation function and consolidates configuration fields into a single `WrenConfiguration` struct stored directly in `WrenVM`, removing separate member fields for reallocate, bindForeignMethod, bindForeignClass, loadModule, minNextGC, and heapScalePercent.

src/cli/vm.c

@@ -145,6 +145,11 @@ static WrenForeignClassMethods bindForeignClass(
   return methods;
 }
 
+static void write(WrenVM* vm, const char* text)
+{
+  printf("%s", text);
+}
+
 static void initVM()
 {
   WrenConfiguration config;
@@ -153,6 +158,7 @@ static void initVM()
   config.bindForeignMethodFn = bindForeignMethod;
   config.bindForeignClassFn = bindForeignClass;
   config.loadModuleFn = readModule;
+  config.writeFn = write;
 
   // Since we're running in a standalone process, be generous with memory.
   config.initialHeapSize = 1024 * 1024 * 100;