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".

Use the `special_argument!` macro to simplify extracting
shell context values from command-line arguments.

Add `add_suggest()` and `add_suggest_with_description()` methods to
`Suggest` for ergonomic batch population of suggestion sets from
collections of strings.

Update the `suggest!` macro to use these new methods and accept
expressions beyond string literals for suggestion items.

methods

Deprecate `ShellContext` methods `filling_argument_first`,
`filling_argument`, `typing_argument`, `strip_typed_argument`, and
`get_typed_arguments` when the `picker` feature is active, as they
do not work under all `ParserStyle` settings.

Add `#![allow(deprecated)]` to the affected modules to suppress
warnings from internal usage.

Extract duplicate `MockCollect` implementations into a reusable
`MockProgramCollect` type. Conditionally require `Serialize` on the
`Groupped` trait when the `general_renderer` feature is enabled.