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.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn getValueBindForeign(const char* signature);