feat: adding json flag to config:get command

[michaelmalave]

Summary

Adds a --json flag to the config:get command that outputs structured JSON, allowing users to distinguish between unset config vars (outputs null) and empty string config vars (outputs ""). This addresses the issue where both cases previously returned empty output, making them indistinguishable. The implementation follows existing patterns in the codebase and maintains full backward compatibility.

Type of Change

• feat: Introduces a new feature to the codebase (minor semvar update)
• fix: Bug fix or issue (patch semvar update)
• perf: Performance improvement
• docs: Documentation only changes
• tests: Adding missing tests or correcting existing tests
• chore: Code cleanup tasks, dependency updates, or other changes

Note: Add a ! after your change type to denote a breaking change.

Testing

Add 2 variables to a new app. One to unset and one as an empty string

# first unset this var
heroku config:unset UNSET_VAR --app safe-dusk-66960

# then run the test commands
./bin/run config:get UNSET_VAR --app safe-dusk-66960 --json
./bin/run config:get EMPTY_STRING_VAR --app safe-dusk-66960 --json

Expected output:

Additional Context

Previously, heroku config:get KEY could not distinguish between:

1. A config var that doesn't exist (unset/removed)
2. A config var that exists but has an empty string value

Both scenarios returned empty output, making it impossible to determine the actual state. The new --json flag provides structured output:

• Unset vars: {"key": "KEY", "value": null}
• Empty string vars: {"key": "KEY", "value": ""}
• Normal vars: {"key": "KEY", "value": "value"}

For multiple keys, the output is an array of objects. The implementation follows the same pattern as config --json and uses hux.styledJSON() for consistent formatting. Default behavior remains unchanged for backward compatibility.

Related Issue

Closes #3450
W-20875855

[eablack]

nice!