refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

rollup.config.js

@@ -0,0 +1,42 @@
+import babel from '@rollup/plugin-babel';
+import terser from '@rollup/plugin-terser';
+import license from 'rollup-plugin-license';
+import path from 'path';
+
+const JS_SRC = '_javascript';
+const JS_DIST = 'assets/js/dist';
+const isProd = process.env.NODE_ENV === 'production';
+
+function build(filename) {
+  return {
+    input: [`${JS_SRC}/${filename}.js`],
+    output: {
+      file: `${JS_DIST}/${filename}.min.js`,
+      format: 'iife',
+      name: 'Chirpy',
+      sourcemap: !isProd
+    },
+    plugins: [
+      babel({
+        babelHelpers: 'bundled',
+        presets: ['@babel/env'],
+        plugins: ['@babel/plugin-proposal-class-properties']
+      }),
+      license({
+        banner: {
+          commentStyle: 'ignored',
+          content: { file: path.join(__dirname, JS_SRC, '_copyright') }
+        }
+      }),
+      isProd && terser()
+    ]
+  };
+}
+
+export default [
+  build('commons'),
+  build('categories'),
+  build('page'),
+  build('post'),
+  build('misc')
+];