[planner] Optimize nodejs final image size (#89)

## Summary
Optimize nodejs final image size by getting rid of dev packages in the
final image.
I'm open to a better schema if postCommandHook is not the best way to go
about it.

Tried it on a fairly complex NodeJS web app:
- size before: 837 MB
- size after (with yarn): 519 MB
- size after (with npm): 515 MB

Sample planner output with yarn:
```
{
  "install_stage": {
    "command": "yarn install",
    "input_files": [
      "package.json",
      "yarn.lock"
    ]
  },
  "build_stage": {
    "command": "yarn build && yarn install --production --ignore-scripts --prefer-offline",
    "input_files": [
      "."
    ],
  },
  "start_stage": {
    "command": "yarn start",
    "input_files": [
      "."
    ]
  },
  "dev_packages": [
    "nodejs",
    "yarn"
  ],
  "runtime_packages": [
    "nodejs",
    "yarn"
  ]
}
```

Sample planner output with npm:
```
"install_stage": {
    "command": "npm install",
    "input_files": [
      "package.json"
    ]
  },
  "build_stage": {
    "command": "npm prune --production",
    "input_files": [
      "."
    ]
  },
  "start_stage": {
    "command": "node index.js",
    "input_files": [
      "."
    ]
  },
  "dev_packages": [
    "nodejs"
  ],
  "runtime_packages": [
    "nodejs"
  ]
```

## How was it tested?
`devbox plan`
`devbox build`

Author: LucilleH

README.md

@@ -143,6 +143,7 @@ Devbox currently detects the following languages:
 
 - Go
 - Python (Poetry)
+- NodeJS
 - PHP
 
 Want more languages? [Ask for a new Language](https://github.com/jetpack-io/devbox/issues) or contribute one via a Pull Request.