Add next_shape_id to vm stats

We need to track this number in CI.  It's important to know how changes
to the codebase impact the number of shapes in the system, so lets add
the number to the VM stat hash

vm.c

@@ -588,7 +588,7 @@ rb_dtrace_setup(rb_execution_context_t *ec, VALUE klass, ID id,
 static VALUE
 vm_stat(int argc, VALUE *argv, VALUE self)
 {
-    static VALUE sym_constant_cache_invalidations, sym_constant_cache_misses, sym_global_cvar_state;
+    static VALUE sym_constant_cache_invalidations, sym_constant_cache_misses, sym_global_cvar_state, sym_next_shape_id;
     VALUE arg = Qnil;
     VALUE hash = Qnil, key = Qnil;
 
@@ -609,6 +609,7 @@ vm_stat(int argc, VALUE *argv, VALUE self)
     S(constant_cache_invalidations);
     S(constant_cache_misses);
         S(global_cvar_state);
+    S(next_shape_id);
 #undef S
 
 #define SET(name, attr) \
@@ -620,6 +621,7 @@ vm_stat(int argc, VALUE *argv, VALUE self)
     SET(constant_cache_invalidations, ruby_vm_constant_cache_invalidations);
     SET(constant_cache_misses, ruby_vm_constant_cache_misses);
     SET(global_cvar_state, ruby_vm_global_cvar_state);
+    SET(next_shape_id, (rb_serial_t)GET_VM()->next_shape_id);
 #undef SET
 
     if (!NIL_P(key)) { /* matched key should return above */