Choose Elixir Language Server

There are multiple language servers in the Elixir ecosystem. This will
allow the user to select which they want to use, with the option of
using more than one.

src/modules/languages/elixir.nix

@@ -13,13 +13,34 @@ in
       default = pkgs.elixir;
       defaultText = lib.literalExpression "pkgs.elixir";
     };
+
+    languageServer = {
+      elixir_ls = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        defaultText = "true";
+        description = ''
+          Enable the ElixirLS language server (https://github.com/elixir-lsp/elixir-ls).
+        '';
+      };
+
+      lexical = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        defaultText = "true";
+        description = ''
+          Enable the Lexical language server (https://github.com/lexical-lsp/lexical).
+        '';
+      };
+    };
   };
 
   config = lib.mkIf cfg.enable
     {
       packages = with pkgs; [
         cfg.package
-        elixir_ls
-      ];
+      ]
+      ++ lib.optional cfg.languageServer.elixir_ls elixir-ls
+      ++ lib.optional cfg.languageServer.lexical lexical;
     };
 }