feat(emacs): Replace LSP-mode with eglot

LSP-mode in combination with the Rust language server does not really
work as RLS is extremely unstable.

This switches over to eglot, which automatically restarts RLS after
every crash. From initial testing it seems to be slightly less
annoying than the previous setup.

packages.nix

@@ -49,7 +49,6 @@ in {
       # Override various Emacs packages from unstable:
       pinnedEmacs = with unstable.emacsPackagesNg; {
         inherit
-          company-lsp
           counsel
           counsel-tramp
           exwm
@@ -57,9 +56,6 @@ in {
           ivy-gitlab
           ivy-hydra
           ivy-pass
-          lsp-mode
-          lsp-rust
-          lsp-ui
           markdown-mode
           markdown-toc
           swiper;