Correct off-by-one word index computation and conditionally pass
option values only when non-empty to avoid spurious parsing.

If compdef is not available, fall back to compctl
directly without invoking an undefined command.

Rework bash completion script to fix word-index tracking when the
cursor is in the middle of a word, compute the current word from
COMP_LINE truncated at COMP_POINT, and use space-separated option
arguments. Also trim colon prefixes from completion items when
COMP_WORDBREAKS includes colons.

Fix zsh completion script to escape colons in completion items
and descriptions, preventing parsing issues with `_describe`.
Also iterate over the original completions array for simple
completions to correctly extract completion items.