`"commandPalette", "launchMode": "commandLine"` doesn't work from the command palette

From: https://github.com/MicrosoftDocs/terminal/issues/578

> The setting:
> `{ "command": "commandPalette", "launchMode": "commandLine", "keys": "" }` does not work for my current version of the terminal (Windows Terminal Version: 1.14.2281.0)

Well, that kinda makes sense. If you're already in the command palette, then toggling the command palette will close it, rather than change the mode (which is what's expected here).

FWIW, binding this to a key does work:

```json
{
"command":
{
"action": "commandPalette",
"launchMode": "commandLine"
},
"keys": "ctrl+shift+."
},
```

I suppose technically, this should move to be a bug on the Terminal repo - the `commandPalette` action with a different mode than the current one should clear the input and switch to that mode. That makes sense.

But alternatively, you could just backspace the `>`, and _presto_, command-line mode.

Start by reproducing the command palette action with launchMode set to commandLine, using the Windows Terminal version and JSON examples in the report. Trace the commandPalette action's handling when the palette is already open and compare it with the keyboard binding behavior. Done means the action switches to command-line mode and clears the existing input instead of closing the palette.

Written by the indexing model from the issue text.