feat: add --watch flag for live file reload

[konaito]

Summary

• Add --watch / -w option that monitors files for changes using the notify crate
• When a watched file is modified externally (e.g. in VS Code, nano, vim), bat automatically clears the screen and re-displays the updated content with full syntax highlighting
• Watches parent directories to correctly handle editors that perform atomic saves (write to temp + rename)
• Includes 100ms debouncing to coalesce rapid filesystem events
• Paging is automatically disabled in watch mode; press Ctrl+C to stop

Usage

bat --watch src/main.rs       # Watch a single file
bat -w config.yaml            # Short form
bat --watch file1.rs file2.rs # Watch multiple files

Motivation

This addresses a common workflow where developers view a file with bat in one terminal while editing it in another (VS Code, nano, vim, etc). Currently, users must manually re-run bat to see changes. With --watch, the display stays up-to-date automatically — similar to tail -f but for the entire file with syntax highlighting.

Closes #852

Test plan

• cargo build succeeds
• cargo test --lib — all 116 tests pass
• bat --watch file.json displays file, updates on external edit, stops on Ctrl+C
• bat --watch without file argument shows helpful error
• bat --help shows --watch flag with description
• Test with editors using atomic saves (VS Code, vim)
• Test with multiple files

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