reflect: implement Value.Grow

This was added in Go 1.20 and becomes necessary for encoding/json in Go
1.21.

src/reflect/all_test.go

@@ -790,6 +790,8 @@ func TestFunctionValue(t *testing.T) {
 	assert(t, v.Type().String(), "func()")
 }
 
+*/
+
 func TestGrow(t *testing.T) {
 	v := ValueOf([]int(nil))
 	shouldPanic("reflect.Value.Grow using unaddressable value", func() { v.Grow(0) })
@@ -857,8 +859,6 @@ func TestGrow(t *testing.T) {
 	})
 }
 
-*/
-
 var appendTests = []struct {
 	orig, extra []int
 }{