Add more demo platforms (#8903)

esphome/components/demo/demo_text.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "esphome/components/text/text.h"
+#include "esphome/core/component.h"
+
+namespace esphome {
+namespace demo {
+
+class DemoText : public text::Text, public Component {
+ public:
+  void setup() override { this->publish_state("I am a text entity"); }
+
+ protected:
+  void control(const std::string &value) override { this->publish_state(value); }
+};
+
+}  // namespace demo
+}  // namespace esphome