Feature request: "--list-settings" flag

Just like `--list` and `--list-submodules` but for listing settings.

Example, for this `justfile`:

```just
set allow-duplicate-recipes
set allow-duplicate-variables
set shell := ["bash", "-euo", "pipefail", "-c"]
set unstable
```

I would get an output like this after running `just --list-settings` (formatting is for demo purposes):

```text
User settings:

- allow-duplicate-recipes: TRUE
- set allow-duplicate-variables: TRUE
- unstable: TRUE
- shell: "bash -euo pipefail -c"

Default settings:
... list of settings with their default values here
```

Start by finding the existing `--list` and `--list-submodules` CLI entry points and their output handling. Trace how user and default settings are represented, then define the listing output for the example settings; done means `just --list-settings` reports both user values and defaults consistently.

Written by the indexing model from the issue text.