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.

Start by inspecting the obfuscate method in the built index.js and the identifierNamesCache option flow. Reproduce the issue with Webpack 5 across multiple dependent files, then verify that the cache is carried between obfuscation results and that the files are obfuscated consistently.

Written by the indexing model from the issue text.