nixd/Controller: Conditionally include devenv option support by default

[k3yss]

This PR adds default support for devenv.sh with the nixd LSP, eliminating the need for manual configuration. This enhancement makes devenv more intuitive and is likely to promote wider adoption of nixd, devenv, and Nix in general. Specifically this PR does the following things:
1: Fetch devenv's option using the devenv-module-options from the flake at https://github.com/cachix/devenv
2: Checks whether the devenv executable is present, if it is not it won't include the worker for options.
3: Loads the LSP in case of devenv executable being present.

Requires: cachix/devenv#1390 to be merged

[inclyc]

Thanks @k3yss, I think it is a good direction to make the whole ecosystem user-friendly.

Also, conflicts should be resolved before merging :)