identifierNamesCache not working as expected

Hi,

I was trying this plugin with WebPack 5 and found that `identifierNamesCache` is not working properly to obfuscate multiple files that depends on each other.

I modified the built `index.js` to fix this issue.

**Old code:**

```js
obfuscate(javascript, fileName, identifiersPrefixCounter) {
const obfuscationResult = javascript_obfuscator_1.default.obfuscate(javascript, Object.assign({ identifiersPrefix: `${WebpackObfuscatorPlugin.baseIdentifiersPrefix}${identifiersPrefixCounter}`, inputFileName: fileName, sourceMapMode: 'separate', sourceMapFileName: fileName + '.map' }, this.options));
return {
obfuscatedSource: obfuscationResult.getObfuscatedCode(),
obfuscationSourceMap: obfuscationResult.getSourceMap()
};
}
```

**New code:**

```js
obfuscate(javascript, fileName, identifiersPrefixCounter) {
const obfuscationResult = javascript_obfuscator_1.default.obfuscate(javascript, Object.assign({ identifiersPrefix: `${WebpackObfuscatorPlugin.baseIdentifiersPrefix}${identifiersPrefixCounter}`, inputFileName: fileName, sourceMapMode: 'separate', sourceMapFileName: fileName + '.map' }, this.options));
this.options.identifierNamesCache = obfuscationResult.getIdentifierNamesCache();
return {
obfuscatedSource: obfuscationResult.getObfuscatedCode(),
obfuscationSourceMap: obfuscationResult.getSourceMap()
};
}
```

I justed add `this.options.identifierNamesCache = obfuscationResult.getIdentifierNamesCache();` and now it works correctly.

PS: I'm not sure if the issue is in this plugin or `javascript-obfuscator` itself.

Thanks.

首先检查构建后的 index.js 中的 obfuscate 方法以及 identifierNamesCache 选项的流程。在多个相互依赖的文件中使用 Webpack 5 复现该问题，然后验证缓存是否在各个混淆结果之间传递，以及这些文件是否被一致地混淆。

由索引模型根据 Issue 内容生成。