feat: add docs for repl and prompt modes

[justinwilaby]

Add REPL and Prompt Command Documentation

Description

This PR adds comprehensive documentation for two interactive features of the Heroku CLI:

1. The REPL (Read-Eval-Print Loop) functionality
2. The interactive prompt functionality

The documentation provides clear guidance on how to use these features and their benefits.

Changes

• Added docs/repl.md with:

  • Command usage and examples
  • Detailed description of REPL capabilities
  • Instructions for using and exiting the REPL
  • Link to source code

• Added docs/prompt.md with:

  • Command usage and examples
  • Detailed description of interactive prompting features
  • Explanation of input validation and guidance
  • Link to source code

• Added supporting command files:

  • src/commands/repl.ts - Guides users to use heroku --repl
  • src/commands/prompt.ts - Guides users to use heroku <COMMAND> --prompt

Why

These documentation additions:

1. Make it easier for users to discover and use interactive features
2. Provide clear guidance on the correct usage patterns
3. Explain the benefits and capabilities of each feature
4. Follow the existing documentation style and format

Testing

• Verify documentation formatting matches other command docs
• Check all links are correct
• Verify examples are accurate and helpful
• Test command help output matches documentation

[justinwilaby]

Thank you @SandyPantsLai !

[tlowrimore-heroku]

This looks good, @justinwilaby! The only question I had has already been answered, that being: why --prompt rather than prompt.