Fix command tree matching when global flags precede the subcommand.

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.

Add `#[completion(EntryFallback)]` syntax to enable custom
completion suggestions for unmatched entries, merging fallback
results with default completions via `Suggest::combine()`.

Rename the internal fallback type and its associated `ProgramCollect`
plumbing from `ErrorDispatcherNotFound`/`build_dispatcher_not_found` to
`EntryFallback`/`build_entry_fallback`, along with the generated enum
variant and pack type. Update all documentation, examples, and tests
accordingly.

Previously, an absent -F and a present -F without a value both
mapped to "unknown". Now, a -F with no value maps to an empty
shell flag, while a missing -F remains "unknown".

Document build-time features, feature pairing, and add a `build.rs`
template for easily setting up `comp` and `pathf`.