Animation of new elements

### Describe the bug

If you use css animation for new elements, the animation will be rendered twice.

### Reproduction

https://github.com/shengxinjing/vue-virtual-scroller/commit/13fafd8487d603fb597cb844b747ca0885feb886

### System Info

```shell
System:
OS: macOS 13.4.1
CPU: (10) arm64 Apple M1 Max
Memory: 2.27 GB / 64.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 18.12.0 - /usr/local/bin/node
Yarn: 1.22.7 - ~/node_modules/.bin/yarn
npm: 8.19.2 - /usr/local/bin/npm
pnpm: 8.6.5 - ~/Library/pnpm/pnpm
bun: 1.0.0 - /usr/local/bin/bun
Browsers:
Chrome: 120.0.6099.129
Safari: 16.5.2
```

### Used Package Manager

pnpm

### Validations

- [X] Read the [docs](https://github.com/Akryum/vue-virtual-scroller/blob/master/packages/vue-virtual-scroller/README.md).
- [X] Check that there isn't [already an issue](https://github.com/Akryum/vue-virtual-scroller/issues) that reports the same bug to avoid creating a duplicate.
- [X] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Akryum/vue-virtual-scroller/discussions).
- [X] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.