feat(niri): add workspace-taskbar support

Adds a per-workspace app-icon taskbar (Workspace class) to the
niri/workspaces module. Rebased onto master: integrated with the
existing window-rewrite feature so the {windows} and {total} format
replacements continue to work alongside the new taskbar.

man/waybar-niri-workspaces.5.scd

@@ -91,6 +91,20 @@ If none of the sorting options are enabled, workspaces keep their output/index o
 	typeof: bool ++
 	default: false ++
 	Enables this module to consume all left over space dynamically.
+	
+*workspace-taskbar*: ++
+	typeof: object ++
+	Contains settings for the workspace taskbar, which displays app icons within each workspace.
+
+		*enable*: ++
+	typeof: bool ++
+	default: false ++
+	Enables the workspace taskbar mode.
+
+		*icon-size*: ++
+	typeof: int ++
+	default: 18 ++
+	Size of the icons in the workspace taskbar.
 
 *ignore-workspaces*: ++
 	typeof: array ++
@@ -166,6 +180,16 @@ Additional to workspace name matching, the following *format-icons* can be set.
 }
 ```
 
+```
+"niri/workspaces": {
+    "format": "{icon}",
+    "workspace-taskbar": {
+        "enable": true,
+        "icon-size": 18
+    }
+}
+```
+
 # Style
 
 - *#workspaces button*
@@ -177,3 +201,5 @@ Additional to workspace name matching, the following *format-icons* can be set.
   the bar that it is displayed on.
 - *#workspaces button#niri-workspace-<name>*: Workspaces named this, or index
   for unnamed workspaces.
+- *#workspaces button.niri-workspace*: The main container for the workspace.
+- *#workspaces button .niri-taskbar-btn*: The icon buttons within the taskbar.