Allow package versions in devbox.json to reference a file

[fvioz]

Summary

Allow package versions in devbox.json to reference a file containing the version string (e.g., .go-version, .node-version). Teams commonly store versions in files to share them across CI, Docker builds, and local development — this ensures devbox environments stay in sync with production without manual duplication.

How was it tested?

1. Create a dummy directory

mkdir dummy

2. Create the following files

dummy/devbox.json

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.17.1/.schema/devbox.schema.json",
  "packages": {
    "go": ".go-version"
  },
  "shell": {
    "init_hook": ["echo 'Welcome to devbox!' > /dev/null"],
    "scripts": {
      "test": ["echo \"Error: no test specified\" && exit 1"]
    }
  }
}

dummy/.go-version

1.24.0

3. Run the following command from the root of the project

devbox run build && cd dummy && ../dist/devbox shell

The expected result should that devbox download the go version 1.24.0 when shell is enabled.

Community Contribution License

All community contributions in this pull request are licensed to the project
maintainers under the terms of the
Apache 2 License.

By creating this pull request, I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 License as stated in
the
Community Contribution License.