Add new options Darwin options:
- `targets.darwin.defaults`
This adds options for configuring macOS through the `defaults(1)` system.
This option can be used to manipulate a vast majority of user settings for macOS
and its applications.
This is implemented using freeform modules and includes additional descriptions
and type information for some useful options.
- `targets.darwin.keybindings`
This adds options for configuring the default keybindings for macOS text fields.
- `targets.darwin.search`
This adds options for configuring the default search engine for macOS.
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