feat(cmd2): improve tab completion for subcommands and flags

Fixed an issue where argparse subcommands and flags were not showing in the prompt-toolkit completion menu on an empty Tab press. Improved Cmd2Completer to accurately calculate the word being completed using cmd2 delimiters. Refactored ArgparseCompleter to reduce complexity and return CompletionItems for flags, providing descriptions in the completion menu. Updated test suite to reflect improved functionality.

Author: tleonhardt