feat: add support for dcm lsp

https://dcm.dev/

lua/lspconfig/server_configurations/dcmls.lua

@@ -0,0 +1,19 @@
+local util = require 'lspconfig.util'
+
+return {
+  default_config = {
+    cmd = { 'dcm', 'start-server' },
+    filetypes = { 'dart' },
+    root_dir = util.root_pattern 'pubspec.yaml',
+  },
+  docs = {
+    description = [[
+https://dcm.dev/
+
+Language server for dart.
+]],
+    default_config = {
+      root_dir = [[root_pattern("pubspec.yaml")]],
+    },
+  },
+}