feat: add WrenValue handle API for persistent object references outside VM

Introduce WrenValue type and wrenGetArgumentValue/wrenReleaseValue functions to allow C code to hold GC-safe references to Wren objects across foreign calls. Implement doubly-linked list management in VM struct, mark value handles during garbage collection, and add assertion in wrenFreeVM to detect unreleased values. Update test infrastructure with new get_value test case and rename existing test binding functions to camelCase convention.

test/api/get_value/get_value.c

@@ -0,0 +1,24 @@
+#include <string.h>
+
+#include "get_value.h"
+
+static WrenValue* value;
+
+static void setValue(WrenVM* vm)
+{
+  value = wrenGetArgumentValue(vm, 1);
+}
+
+static void getValue(WrenVM* vm)
+{
+  wrenReturnValue(vm, value);
+  wrenReleaseValue(vm, value);
+}
+
+WrenForeignMethodFn getValueBindForeign(const char* signature)
+{
+  if (strcmp(signature, "static Api.value=(_)") == 0) return setValue;
+  if (strcmp(signature, "static Api.value") == 0) return getValue;
+
+  return NULL;
+}