Add file sync option to zip command

[pkova]

Fixes #84

The problem can be reproduced by running npm run release, modifying some code, running npm run release again and inspecting the .zip.

You'll get something like

tree -D build
build
├── [Apr  2 17:09]  asset-manifest.json
├── [Mar 18 22:53]  favicon.ico
├── [Apr  2 17:09]  index.html
├── [Mar 18 22:53]  manifest.json
├── [Apr  2 17:09]  precache-manifest.4eb231253bd00e4e654613b55221e927.js
├── [Apr  2 17:07]  precache-manifest.955df6be55bd39037f7132152208d8ce.js
├── [Apr  2 17:09]  service-worker.js
└── [Apr  2 17:09]  static
    ├── [Apr  2 17:09]  css
    │   ├── [Apr  2 17:09]  main.17bd1ca5.chunk.css
    │   └── [Apr  2 17:09]  main.17bd1ca5.chunk.css.map
    ├── [Apr  2 17:09]  js
    │   ├── [Apr  2 17:09]  1.7700c553.chunk.js
    │   ├── [Apr  2 17:09]  1.7700c553.chunk.js.map
    │   ├── [Apr  2 17:09]  main.32589c19.chunk.js
    │   ├── [Apr  2 17:09]  main.32589c19.chunk.js.map
    │   ├── [Apr  2 17:07]  main.c99d9132.chunk.js
    │   ├── [Apr  2 17:07]  main.c99d9132.chunk.js.map
    │   ├── [Apr  2 17:09]  runtime~main.229c360f.js
    │   └── [Apr  2 17:09]  runtime~main.229c360f.js.map
    └── [Apr  2 17:09]  media
        ├── [Apr  2 17:09]  Inter-UI-Medium.5292a06c.woff
        ├── [Apr  2 17:09]  Inter-UI-Medium.63ba5137.woff2
        ├── [Apr  2 17:09]  Inter-UI-Regular.b36fe2ea.woff
        ├── [Apr  2 17:09]  Inter-UI-Regular.d74d59d2.woff2
        ├── [Apr  2 17:09]  Inter-UI-SemiBold.462a99bc.woff
        └── [Apr  2 17:09]  Inter-UI-SemiBold.a2ae283d.woff2

Where the hashes are different and therefore some files get added twice.

Adding the file sync option to the zip command fixes this.

[alexmatz]

Thanks for your contribution @pkova! Much appreciated and delighted to have you involved :)

[Fang-]

Sweet, this is a really nice fix. Thanks!