Make emulated grep support --color argument

[UdeshyaDhungana]

• Add feature for Grep/Highlighting extension in emulated grep

---

Note

Adds --color (always|never|auto) to emulated grep with TTY-aware match highlighting and updates tests/utilities accordingly.

• Grep core:
  • Add --color flag with modes always|never|auto and validate input.
  • Introduce EmulationOptions{ Stdin, EmulateInTTY } and update EmulateGrep(args, EmulationOptions) signature.
  • Enable match highlighting for stdin searches based on color mode and EmulateInTTY.
• Highlighting:
  • Implement highlightMatches in internal/grep/stdin_search.go using utils.HighlightString for ANSI-wrapped matches.
• Tests:
  • Extend stdin tests to cover color modes, TTY behavior, and multi-match highlighting.
  • Update test harnesses to pass EmulationOptions and compare against new highlighting behavior.
• Utils:
  • Add utils.HighlightString for grep-style ANSI highlighting.

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[rohitpaulk]

Try to avoid logic in assertions - hardcode the expected bytes here. That way we're protected against accidental changes to the logic inside utils.HighlightString.