niri/workspaces: Allow restricting the module to an output

[algernon]

To be able to show the workspaces for a given output only, which may be different from the output the bar is displayed on, allow restricting the niri/workspaces module to a single output.

The default - when the output option is unset - remains the same as before.

This lets me set up Waybar to do this:

The "main" and "social" modules on the left are configured like this:

    "niri/workspaces#left": {
        "current-only": true,
        "disable-click": true,
        "output": "DP-1",
        "format": "🄻 {value}"
    },
    "niri/workspaces#right": {
        "current-only": true,
        "disable-click": true,
        "output": "HDMI-A-1",
        "format": "🅁 {value}"
    },

In other words, this lets me show the active workspace on both my displays, on a single bar.