Replace bool-based settings with enums, refine visibility and
signatures, and standardize `Self` usage across the crate.

- Introduce `ErrorOutput`, `RenderOutput`, `PanicSilence`, `Verbosity`,
  `ColorOutput`, and `ProgressOutput` enums for clearer configuration
  semantics
- Make `GlobalResources`, `ProgramCell`, and
  `split_input`/`split_input_string` public
- Change hook info parameters to accept references and
  `split_input_string` to take `&str`
- Apply `Self` shorthand throughout implementations and add
  `#[must_use]` attributes where appropriate
- Enable strict clippy lints with targeted allowances

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.

Fix the method to use `eprintln!` instead of `println!` when
`immediate_output` is enabled, ensuring error output goes to stderr.

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