Add Ctrl+p / Ctrl+n support for previous/next in debug repl

[rlivings39]

In the debug repl I frequently want Ctrl+p / Ctrl+n to navigate previous / next in the history. We can do this with:

    {
        "key": "ctrl+p",
        "command": "history.showPrevious",
        "when": "inDebugRepl",
    },
    {
        "key": "ctrl+n",
        "command": "history.showNext",
        "when": "inDebugRepl",
    }

Could those be added?