A separate command for remapping all kinds of key shortcuts Code supports

Our current remappers can only map keystorkes we registered in package.json but we can't do this for all kinds of key shortcuts. However we can provide a command to allow users to do that, by leveraging arguments of keybindings. For example

    {
        "key": "F3",
        "command": "vim.remap",
        "when": "editorTextFocus",
        "args": {
            "value": ["a", "b", "c", "<Esc>"]
        }
    }

F3 can enter Insert Mode, insert bc and quit Insert Mode. And we don't need to make our package.json dirty.