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/widgets/settingspages/SettingWidget.hpp

@@ -23,7 +23,7 @@ namespace chatterino {
 
 class GeneralPageView;
 
-class SettingWidget : QWidget
+class SettingWidget : public QWidget
 {
     Q_OBJECT
 
@@ -66,6 +66,11 @@ public:
                                    QStringSetting &setting,
                                    const QString &placeholderText = {});
 
+    static SettingWidget *fontButton(const QString &label,
+                                     QStringSetting &familySetting,
+                                     std::function<QFont()> currentFont,
+                                     std::function<void(QFont)> onChange);
+
     SettingWidget *setTooltip(QString tooltip);
     SettingWidget *setDescription(const QString &text);