[Feature Request] Support any unmodified SearXNG instance

[Kamilkampfwagen-II]

Currently the addon only supports the former SearX, and even then it doesn't work with instances whose url isn't in the list (I think).
SearX is no longer maintained, it's fork SearXNG is.
The addon could allow for a custom instances list, thus allowing even private ones. Most instances out there is not modified anyway.