Add comprehensive help message and -h flag support

[markparonyan]

Summary

This change adds a dedicated help function and implements the -h flag to display detailed usage information about dvtm, improving user experience for new users seeking command documentation.

Key Changes

• New help() function: Displays comprehensive help text including:

  • General usage description
  • Detailed option documentation
  • Common keybindings reference
  • Reference to the full manual

• Updated usage() function: Now directs users to run dvtm -h for more information

• Argument parsing enhancement: Modified parse_args() to recognize both --help and -h flags (when -h is the last argument without a value), calling the new help() function instead of usage()