Fix parseModuleJson undefined call in custom command modules

CustomCommandModule.update() in Bar.qml called root.parseModuleJson(raw)
but the function was removed in 3b970068 (the indicator split). Any
custom command module emitting waybar-style JSON would have crashed on
first poll.

Hoist the helper into Util so both call sites — the bar's custom command
module and BarIndicator.extractData — share one definition.

shell/plugins/bar/Bar.qml

@@ -751,7 +751,7 @@ Item {
     }
 
     function update(raw) {
-      var data = root.parseModuleJson(raw)
+      var data = Util.parseModuleJson(raw)
       var klass = data.class || data.alt || ""
 
       outputText = data.text || String(raw || "").trim()