Reaction of click web-ext popup icon.

use popup.html

in `manifest.json`

{
  "browser_action": {
    "default_icon": "./assets/icon-512.png"
    "default_popup": "./dist/popup/index.html",
  }
}

browserAction API

in `background.ts`, if you use `webextension-polyfill`, `browser` will be accessed from global namespace.

browser.browserAction.onClicked.addListener(function(tab) {
  alert("working?");
});

1. 打开`chrome-devtools`
2. 点击响应式视图（那个手机小图标）
3. 设置为`1280 * 800`

适合复制那些通过`console.log`打印得到变量，这些变量往往没有赋值。关键在于copy指令