Add userscripts support

### Problem Description

The standard version of the extension does not support AdGuard Extra, which makes it impossible to get rid of ads on some popular sites. Users have to install a separate extension to support custom scripts.

### Proposed Solution

Add custom script support to the extension.

### Alternatives Considered

Make a separate AdGuard Extra extension and submit it in the App Store.

### Additional Information

https://github.com/AdguardTeam/AdGuardExtra/issues/311