Fix stdin reading bug and upgrade to convert_case 0.8 from 0.6

[BernardIgiri]

This PR addresses multiple issues and enhancements related to CLI behavior and library compatibility:

🐛 Bug Fix

• Fixes [#2](https://github.com/rutrum/ccase/issues/2): Correctly detects when stdin is piped, allowing input fallback to work reliably in all environments.

🚀 Upgrade

• Upgrades convert_case from 0.6 to 0.8, which introduces API-breaking changes:
  • The Pattern enum was removed.
  • Case now uses Case<'a> with Custom as a structured variant.
  • Boundary handling API changed slightly.

🛠️ Refactor

• Introduces UserCase and UserPattern enums as intermediaries:
  • Provides a stable internal abstraction over convert_case changes
  • Uses strum macros to support:
    • CLI parsing via EnumString
    • Help output via Display and VariantNames
    • Iteration via EnumIter
• CLI help output now mirrors the original style with aligned and styled case/pattern listings.

✅ Tests

• Made the previously ignored test input_required runnable using an environment flag
• Added new tests for:
  • Invalid case and pattern inputs (e.g., SNEK, SENT)
  • Pattern application with custom delimiter (-p camel -d ++)
  • Help output includes all expected cases and patterns
  • Input args override stdin when both are present

📌 Compatibility

• Maintains backwards-compatible CLI behavior for all existing options
• Prepares for future enhancements by decoupling internal representation from upstream API