feat: refactor dart and elixir LSPs according to the new structure

modules/languages/dart/config.nix

@@ -0,0 +1,60 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+with lib;
+with builtins; let
+  cfg = config.vim.languages.dart;
+  ftcfg = cfg.flutter-tools;
+  servers = {
+    dart = {
+      package = pkgs.dart;
+      lspConfig = ''
+        lspconfig.dart.setup{
+          capabilities = capabilities;
+          on_attach=default_on_attach;
+          cmd = {"${pkgs.dart}/bin/dart"};
+          ${optionalString (cfg.lsp.opts != null) "init_options = ${cfg.lsp.dartOpts}"}
+        }
+      '';
+    };
+  };
+in {
+  config = mkIf cfg.enable (mkMerge [
+    (mkIf cfg.treesitter.enable {
+      vim.treesitter.enable = true;
+      vim.treesitter.grammars = [cfg.treesitter.package];
+    })
+
+    (mkIf cfg.lsp.enable {
+      vim.lsp.lspconfig.enable = true;
+
+      vim.lsp.lspconfig.sources.dart-lsp = servers.${cfg.lsp.server}.lspConfig;
+    })
+
+    (mkIf (ftcfg.enable) {
+      vim.startPlugins = ["flutter-tools"];
+
+      vim.luaConfigRC.flutter-tools = nvim.dag.entryAnywhere ''
+        require('flutter-tools').setup {
+          lsp = {
+            color = { -- show the derived colours for dart variables
+              enabled = ${boolToString ftcfg.color.enable}, -- whether or not to highlight color variables at all, only supported on flutter >= 2.10
+              background = ${boolToString ftcfg.color.highlightBackground}, -- highlight the background
+              foreground = ${boolToString ftcfg.color.highlightForeground}, -- highlight the foreground
+              virtual_text = ${boolToString ftcfg.color.virtualText.enable}, -- show the highlight using virtual text
+              virtual_text_str = ${ftcfg.color.virtualText.character} -- the virtual text character to highlight
+            },
+
+            capabilities = capabilities,
+            on_attach = default_on_attach;
+            flags = lsp_flags,
+          },
+        }
+
+      '';
+    })
+  ]);
+}