[gen readme] Implement gen readme command

[mikeland73]

Summary

Implements devbox gen readme [filename] command. See devbox.md for what the output looks like.

Highlights:

• Adds scripts with local links
• Comments in devbox.json scripts are added to readme. (Limited support for these. Comments after script name and before script value are supported)
• Adds packages and links to URL (nixhub or github)
• Adds comment indicating generated by devbox. This comment can be used to insert readme into an existing readme.

How was it tested?

devbox gen readme devbox.md

[mikeland73]

@gcurtis @loreto PTAL

This PR is not urgent, but don't want it to get stale.

[loreto]

LGTM

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ **Generic Error: <redacted *errors.withStack>: <redacted errors.withStack>: <redacted fmt.wrapError>: <redacted ... go.jetpack.io/devbox/internal/devbox in Open View Issue

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