chore: replace serve, watch and concurrently with node --watch and npx

node --watch covers the rebuild side, including the template inside
node_modules when the path is given explicitly, so build:watch and the
watch package can go. http-server runs through npx, so the serving side
needs no dependency either, and because it is one script with two
children concurrently is not needed.

npm run serve rebuilds on changes in lib/, docs/ and the jsdoc template
and serves build/ on 127.0.0.1.

Author: RobinBol

eslint.config.js

@@ -28,6 +28,12 @@ module.exports = defineConfig(
       ],
     },
   },
+  {
+    files: ['**/*.mjs'],
+    languageOptions: {
+      sourceType: 'module',
+    },
+  },
   {
     files: ['test/**/*.js'],
     languageOptions: {