Introduce `might_be_async` as a workspace dependency, add it to
`mingling_core`, and regenerate all Cargo.lock files accordingly.
Multiple examples and test packages also pick up a newly transitive
`equivalent` crate.

Replace the `builds` feature with `build` across the crate, keeping
a backward-compatible alias for the old name.  Update all internal
visibility from `#[doc(hidden)] pub mod` to `pub(crate) mod` and
reorganize the `__private` module into a dedicated `private.rs`.

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.