`set -S` should mark read-only variables

fish 3.3.1

Most special variables defined by fish are read-only. `set -S` does not indicate this in any way. For example:

```
 ❯ set -S PWD
$PWD: set in global scope, exported, with 1 elements
$PWD[1]: |/Users/lily|
```

This should instead look something like

```
 ❯ set -S PWD
$PWD: set in global scope, exported, read-only, with 1 elements
$PWD[1]: |/Users/lily|
```