Prototype Pollution in the plugin jss-plugin-extend

__Expected behavior:__
The prototype object `Object.prototype` should not be modified.

__Describe the bug:__
There is a prototype pollution vulnerability in the plugin jss-plugin-extend

__Reproduction:__

Using the following exploit:

```
var jss = require('jss');
var extend = require('jss-plugin-extend');

var s = jss.create()
.use(extend.default())
.createStyleSheet(
{extend: {extend: {constructor: {prototype: {polluted: 'yes'}}}}}
);
console.log(Object.prototype.polluted === 'yes')
```

__Versions (please complete the following information):__
- jss: 10.10.0 (newest)
- OS [Linux, macOS, Windows]:

Start by locating the jss-plugin-extend implementation and running the supplied reproduction to confirm that Object.prototype is modified. Trace how the nested extend input is handled, then verify the fix against the same exploit; done means Object.prototype remains unchanged.

Written by the indexing model from the issue text.