… arguments

In some cases in VSCode + fish you can run a command like:

```
 echo 'hi                                                                                                               
                -- hi'
```

It's perfectly legitimate and will run in a normal terminal just fine.

However VSCode's shell extensions will raise an error:

```
string join: -- hi': unknown option

/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/contrib/terminal/browser/media/fish_xdg_data/fish/vendor_conf.d/shellIntegration.fish (line 1): 
string join ";" $argv
^
in command substitution
        called on line 65 of file /Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/contrib/terminal/browser/media/fish_xdg_data/fish/vendor_conf.d/shellIntegration.fish
in function '__vsc_esc' with arguments 'E echo\ \'hi "" --\ hi\''
        called on line 72 of file /Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/contrib/terminal/browser/media/fish_xdg_data/fish/vendor_conf.d/shellIntegration.fish
in function '__vsc_cmd_executed' with arguments 'echo\ \'hi\n\n--\ hi\''
in event handler: handler for generic event 'fish_preexec'

(Type 'help string' for related documentation)
```

The problem is `string` interprets the `--hi` as a switch.  By adding `--` we can negate that behavior and let `--hi` be treated as a positional argument.

See the [string documentation][s]

> Arguments beginning with - are normally interpreted as switches; -- causes the following arguments not to be treated as switches even if they begin with -. Switches and required arguments are recognized only on the command line.

[s]: https://fishshell.com/docs/current/cmds/string.html