Update to Lit 2.0.0 (manually)

Will attempt to get this back to syncing with monorepo history per https://github.com/lit/lit#exporting-starter-templates in a follow-up.

web-dev-server.config.js

@@ -6,9 +6,15 @@
 
 import {legacyPlugin} from '@web/dev-server-legacy';
 
+const mode = process.env.MODE || 'dev';
+if (!['dev', 'prod'].includes(mode)) {
+  throw new Error(`MODE must be "dev" or "prod", was "${mode}"`);
+}
+
 export default {
-  nodeResolve: true,
+  nodeResolve: {exportConditions: mode === 'dev' ? ['development'] : []},
   preserveSymlinks: true,
+  rootDir: 'docs',
   plugins: [
     legacyPlugin({
       polyfills: {