feat: migrate heroku cli command diff to core cli

[michaelmalave]

Summary

This migrates the Heroku command apps:diff from the separate plugin (@heroku/heroku-cli-diff) into the core CLI. The command is converted to use oclif/core and ESM, and unit tests are added.

Type of Change

Breaking Changes (major semver update)

• Add a ! after your change type to denote a change that breaks current behavior

Feature Additions (minor semver update)

• feat: Introduces a new feature to the codebase

Patch Updates (patch semver update)

• fix: Bug fix
• deps: Dependency upgrade
• revert: Revert a previous commit
• chore: Change that does not affect production code
• refactor: Refactoring existing code without changing behavior
• test: Add/update/remove tests

Testing

Notes:

CI tests should pass

Steps:

1. npm run build
2. ./bin/run apps:diff $App1 $App2

Screenshots (if applicable)

Related Issues

GitHub issue: #[GitHub issue number]
GUS work item: W-17818328