Fix options.extensions being modified by the plugin

[Sidnioulz]

Hi,

Your plugin modifies the extensions object passed to its options, which means it cannot be reused across plugins. This means the following sample config fails to properly resolve imports:

const cleanup = require('rollup-plugin-cleanup')
const resolve = require('@rollup/plugin-node-resolve').default

const extensions = ['.js', '.jsx', '.ts', '.tsx', '.json', '.md', '.mdx']

module.exports = {
	// ...
	plugins: [
		resolve({ extensions }),
		cleanup({ extensions }),
	],
}

The following PR will ensure extensions are copied instead of written over. It allows Rollup users to share an extension list across plugins.

[Sidnioulz]

I don't know what's up with the build bot. The yarn build and yarn test commands both succeed locally.