Add stretching of modules and modules-center toggling

This Patch allows the stretching of modules-{left,center,right} as well
add a "expand" flag to AModule. This allows one module to consume the
leftover space.
To allow the left or right modules to fully consume the center, the
changes also include a way to remove the center box (center_)
altogether.

man/waybar-niri-workspaces.5.scd

@@ -45,6 +45,11 @@ Addressed by *niri/workspaces*
 	typeof: string ++
 	Command to execute when the module is updated.
 
+*expand*: ++
+	typeof: bool ++
+	default: false ++
+	Enables this module to consume all left over space dynamically.
+
 # FORMAT REPLACEMENTS
 
 *{value}*: Name of the workspace, or index for unnamed workspaces,