Implement initial config loading

Config goes in $XDG_CONFIG_HOME/beansprout/config.kdl or
$HOME/.config/beansprout/config.kdl

Config is in the kdl format. Right now, the supported options are

```zig
/// Width of window borders in pixels
border_width: u8 = 2,
/// Color of focused window's border in 0xRRGGBBAA or 0xRRGGBB form
border_color_focused: RiverColor = utils.parseRgbaComptime("0x89b4fa"),
/// Color of uffocused windows' borders in 0xRRGGBBAA or 0xRRGGBB form
border_color_unfocused: RiverColor = utils.parseRgbaComptime("0x1e1e2e"),

/// Where a new window should attach, top or bottom of the stack
attach_mode: AttachMode = .top,
/// Should focus change when the cursor moves onto a new window
focus_follows_pointer: bool = true,
/// Should the pointer warp to the center of newly-focused windows
pointer_warp_on_focus_change: bool = true,
```

I plan to add Keybinds shortly. If parsing the configuration fails,
the default config will be used and the WM will continue loading.

src/Output.zig

@@ -56,8 +56,6 @@ fn outputListener(river_output_v1: *river.OutputV1, event: river.OutputV1.Event,
             output.height = ev.height;
         },
         .position => |ev| {
-            // TODO - CONFIG: Allow setting output position (do I even need to
-            // do this, or do I just get told what the position is?)
             output.x = ev.x;
             output.y = ev.y;
         },