feat: add custom elements language server support (#2570)

lua/lspconfig/server_configurations/custom_elements_ls.lua

@@ -0,0 +1,64 @@
+local util = require 'lspconfig.util'
+
+local bin_name = 'custom-elements-languageserver'
+local cmd = { bin_name, '--stdio' }
+
+if vim.fn.has 'win32' == 1 then
+  cmd = { 'cmd.exe', '/C', bin_name, '--stdio' }
+end
+
+return {
+  default_config = {
+    init_options = { hostInfo = 'neovim' },
+    cmd = cmd,
+    filetypes = {
+      'javascript',
+      'javascriptreact',
+      'javascript.jsx',
+      'typescript',
+      'typescriptreact',
+      'typescript.tsx',
+      'html',
+    },
+    root_dir = function(fname)
+      return util.root_pattern 'tsconfig.json'(fname)
+        or util.root_pattern('package.json', 'jsconfig.json', '.git')(fname)
+    end,
+  },
+  docs = {
+    description = [[
+https://github.com/Matsuuu/custom-elements-language-server
+
+`custom-elements-languageserver` depends on `typescript`. Both packages can be installed via `npm`:
+```sh
+npm install -g typescript custom-elements-languageserver
+```
+To configure typescript language server, add a
+[`tsconfig.json`](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html) or
+[`jsconfig.json`](https://code.visualstudio.com/docs/languages/jsconfig) to the root of your
+project.
+Here's an example that disables type checking in JavaScript files.
+```json
+{
+  "compilerOptions": {
+    "module": "commonjs",
+    "target": "es6",
+    "checkJs": false
+  },
+  "exclude": [
+    "node_modules"
+  ]
+}
+
+The best way to utilize the Custom Elements Language Server is to enable the [Custom Elements Manifest](https://github.com/webcomponents/custom-elements-manifest)(CEM) in your project by installing
+a CEM generator like one provided by [The Open WC Team](https://github.com/open-wc/custom-elements-manifest/tree/master/packages/analyzer).
+
+Generating a CEM in watch mode will provide you with the best user experience. If your dependencies ship with a Custom Elements Manifest, those will be utilized also.
+
+```
+]],
+    default_config = {
+      root_dir = [[root_pattern("package.json", "tsconfig.json", "jsconfig.json", ".git")]],
+    },
+  },
+}