Add breaking change for `with_dispatcher` removal, renumber feature
items, and outline automated `dispatcher_tree` optimization plan.

Update changelog and issue tracker notes for the enhanced
`default_completion` behavior.

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

Update all references across docs, examples, and configuration files to
use the shorter `extras` feature name.