neovim: add treesitter and lsp config (#3129)

see utils/nvim/README.md

utils/nvim/carbon.lua

@@ -0,0 +1,28 @@
+-- Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+-- Exceptions. See /LICENSE for license information.
+-- SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+vim.filetype.add({
+  extension = {
+    carbon = 'carbon',
+  },
+})
+
+vim.treesitter.language.add("carbon")
+
+-- LSP
+local lsp = require 'lspconfig'
+local configs = require 'lspconfig.configs'
+local util = require 'lspconfig.util'
+
+-- Check if the config is already defined (useful when reloading this file)
+if not configs.carbon then
+  configs.carbon = {
+    default_config = {
+      cmd = { "./bazel-bin/language_server/language_server" },
+      filetypes = { "carbon" },
+      root_dir = util.find_git_ancestor,
+    }
+  }
+end
+lsp.carbon.setup {}