feat(html,css): add HTML and CSS language servers

HTML and CSS were formatted by Prettier but had no language server
apart from Tailwind's. Add vscode-html-language-server and
vscode-css-language-server, which also covers SCSS and Less, plus the
SCSS Treesitter parser.

lua/plugins/html-css.lua

@@ -0,0 +1,15 @@
+return {
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = { ensure_installed = { "html", "css", "scss" } },
+  },
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        html = {},
+        cssls = {},
+      },
+    },
+  },
+}