feat: add REPL for plugins (#6120)

This can be enabled by setting the `.plugins.repl.enabled` setting to true


Close Chatterino and make a backup of your settings.json before attempting any modifications.
```json5
{
  "plugins": {
    "repl": {
      "enabled": true,
    },
  },
}
```

src/CMakeLists.txt

@@ -594,6 +594,8 @@ set(SOURCE_FILES
         widgets/Notebook.hpp
         widgets/OverlayWindow.cpp
         widgets/OverlayWindow.hpp
+        widgets/PluginRepl.cpp
+        widgets/PluginRepl.hpp
         widgets/Scrollbar.cpp
         widgets/Scrollbar.hpp
         widgets/TooltipEntryWidget.cpp