Option `--icon` with absent mode argument should be equivalent to `--icon always`

Nobody has claimed this yet.

# existing behavior

the `--icon` option requires an argument

````
~ $ lsd --icon
error: a value is required for '--icon ' but none was supplied
[possible values: always, auto, never]
~ $
````

# expected behavior

the `--icon` option with no argument should enable icons

````
~ $ lsd --icon
📂 Desktop 📂 Developer 📂 Documents 📂 Downloads 📂 Library 📂 Movies 📂 Music 📂 Pictures 📂 Public
~ $
````

# details

it is reasonable to assume that manual application of an `--icon` option with no arguments intends `--icon always`

in my `~/.config/lsd/config.yml` i have

````
icons:
when: never
theme: unicode
````

i have many short aliases defined in my `.zshrc` for various modes and behaviors, none of them involving icons.

occasionally, i might want to display icons. it would be nice to simply run `lsd --icon` (or any of my aliases, with an icon option) to activate icon display.

i understand this feature might have been deliberately avoided due to the possibility for accidental application of following file name arguments or expanded glob values following a bare `--icon` option. this type of mistake is already a well-understood possibility, and the broadly accepted solution is a standard `--` options-terminating sentinel.

Start by locating the CLI definition for `--icon` and any existing tests covering option parsing. Reproduce the current missing-argument error, then verify that a bare `--icon` enables icons while preserving the configured icon theme and other existing modes.

Written by the indexing model from the issue text.