fix unstable plugin reference with flat configs

The previous use of spread meant that
```ts
import jsxA11y from 'eslint-plugin-jsx-a11y';

jsxA11y !== jsxA11y.flatConfigs.recommended.plugins['jsx-a11y']
```

This is a problem because if someone does something like this 
```js
import jsxA11y from 'eslint-plugin-jsx-a11y';

export default [
  { plugins: { 'jsx-a11y': jsxA11y } },
  jsxA11y.flatConfigs.recommended,
];
```
then ESLint will crash with the error `Config "jsx-a11y/recommended": Key "plugins": Cannot redefine plugin "jsx-a11y".`

This PR fixes that by using `Object.assign` to mutate the `jsxA11y` object and maintain referential equality.

Author: bradzacher

src/index.js

@@ -317,4 +317,4 @@ const flatConfigs = {
   strict: createConfig(strictRules, 'strict'),
 };
 
-module.exports = { ...jsxA11y, configs, flatConfigs };
+module.exports = Object.assign(jsxA11y, { configs, flatConfigs });