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/include/wren.h

@@ -48,6 +48,9 @@ typedef WrenForeignMethodFn (*WrenBindForeignMethodFn)(WrenVM* vm,
                                                        bool isStatic,
                                                        const char* signature);
 
+// Displays a string of text to the user.
+typedef void (*WrenWriteFn)(WrenVM* vm, const char* text);
+
 typedef struct
 {
   // The callback invoked when the foreign object is created.
@@ -111,6 +114,12 @@ typedef struct
   // stored in the foreign object when an instance is created.
   WrenBindForeignClassFn bindForeignClassFn;
 
+  // The callback Wren uses to display text when `System.print()` or the other
+  // related functions are called.
+  //
+  // If this is `NULL`, Wren discards any printed text.
+  WrenWriteFn writeFn;
+  
   // The number of bytes Wren will allocate before triggering the first garbage
   // collection.
   //