refactor(core): split web entry from core (#6082)

This pr is trying to split `web` and `electron` entries from `core`. It allows more platform-related optimization to be addressed in each entry.
We should remove all browser/electron only codes from `core` eventually, this is the very first step for that.

tools/cli/src/webpack/postcss.config.cjs

@@ -0,0 +1,20 @@
+const cssnano = require('cssnano');
+
+module.exports = function (context) {
+  const plugins = [
+    cssnano({
+      preset: [
+        'default',
+        {
+          convertValues: false,
+        },
+      ],
+    }),
+  ];
+
+  return {
+    from: context.from,
+    plugins,
+    to: context.to,
+  };
+};