Request: add support for LibreTranslate's API

**Is your feature request related to a problem? Please describe.**
I'm a self-hoster, and already have a local translation service powered by LibreTranslate. I would like to use Phanpy for my local server, but do not wish to use Lingva as the translation API, as that requires connecting to an external Google server that could read my messages.

**Describe the solution you'd like**
I would like for Phanpy to be optionally compatible with LibreTranslate's API, perhaps by using [lingva-api](https://github.com/cheeaun/lingva-api) as the common backend.

**Describe alternatives you've considered**
As of now there are two alternatives: installing and using Lingva in my own server, along with lingva-api, which defeats the purpose of using a self-hosted translation service; and not supplying a translation service at all on my local version of Phanpy.