Rephrasing with DeepL

### Describe the problem

DeepL now has https://developers.deepl.com/api-reference/improve-text which could be used for rephrasing. We already have this feature for LLM-based machineries, so this might be the time to formalize that.

### Solution brainstorm

* Add base form with toggling rephrasing support.
* Add base machinery class/mixin with rephrasing support (if needed).
* Adjust LLM-based machineries to rephrase only when turned on.
* Implement rephrasing for DeepL, keep in mind that it only supports subset of languages currently.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_