Gmail add-on: Copy Webpack plugin. Hook after emit.

webpack_v4.config.js

const exec = require("child_process").exec;
[...]
module.exports = {
 [...]
  plugins: [
    [...]
    new CopyWebpackPlugin([
      { from: join(root, "config/dev/.clasp.json"), to: ".", ToType: "file" },
    ]),
    {
      apply: compiler => {
        compiler.hooks.afterEmit.tap("AfterEmitPlugin", () => {
          exec("yarn push", (_, stdout, stderr) => {
            if (stdout) process.stdout.write(stdout);
            if (stderr) process.stderr.write(stderr);
          });
        });
      },
    },
  ]
}