Implement `Pickable` for `String` supporting both positional and named
flags, and generalize value matching to use style-specific separators

Deduplicate the `seek_end_of_options` function by moving it from both
`arg_matcher` and `flag_matcher` into a single shared implementation in
utils. Add `make_masked` and `make_args` test helpers to the test crate

Implement `on_match_one` and `on_match_all` for `ArgMatcher`,
supporting named flags (with eq-mode and short flags), positional
arguments, end-of-options marker (`--`), and case-insensitive matching

iteration

Add a new `Flag` enum in `mingling_picker::value` that explicitly
distinguishes between an absent flag (`Inactive`) and a present flag
(`Active`), along with its `Pickable` implementation.  Rename internal
fields from `flag_*` to `arg_*` for consistency, update the default
naming case to `Kebab`, and enable the `mingling_support` feature in
workspace settings.

PickerArgAttr

Rename `bool_matcher` module to `flag_matcher` and introduce a dedicated
`FlagMatcher` struct, moving matching logic away from the `bool` type
itself