Feature: Add new backend: nix

[PhilTaken]

This PR introduces a new optional backend to pull the language servers from: nix. Nix does not necessarily fit the containerization theme of this plugin, but provides a very similar user experience to the docker backend. It also cuts down on the (albeit very slim) containerization overhead.

The nix backend provides the same interface as the docker/podman approach in that it can pull/build and delete language servers dynamically to save on disk space.

Since this is quite a big change relatively speaking, I consider this more of a draft for now.

[WhyNotHugo]

nix-provided LSPs can be sandboxed with bubblewrap. I used this briefly and it worked pretty well. I eventually moved from nix to just using my distro's packages, so don't have many references on it.