Support alwaysOn: false, and an explicit command

[tmcw]

This makes it possible to turn off "codeium all the time" and manually trigger it instead. If you install and run npm run dev, you can open up a demo of this mode where you have to hit cmd-k to trigger completion. There's no default explicit trigger - it can be added with a keymap like

keymap.of([
      {
        key: "Cmd-k",
        run: startCompletion,
      },
    ]),