[button] Convert to C++17 nested namespace style (#12233)

Co-authored-by: Keith Burzinski <kbx81x@gmail.com>

esphome/components/button/automation.h

@@ -4,8 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 
-namespace esphome {
-namespace button {
+namespace esphome::button {
 
 template<typename... Ts> class PressAction : public Action<Ts...> {
  public:
@@ -24,5 +23,4 @@ class ButtonPressTrigger : public Trigger<> {
   }
 };
 
-}  // namespace button
-}  // namespace esphome
+}  // namespace esphome::button

esphome/components/button/button.cpp

@@ -1,8 +1,7 @@
 #include "button.h"
 #include "esphome/core/log.h"
 
-namespace esphome {
-namespace button {
+namespace esphome::button {
 
 static const char *const TAG = "button";
 
@@ -26,5 +25,4 @@ void Button::press() {
 }
 void Button::add_on_press_callback(std::function<void()> &&callback) { this->press_callback_.add(std::move(callback)); }
 
-}  // namespace button
-}  // namespace esphome
+}  // namespace esphome::button

esphome/components/button/button.h

@@ -4,8 +4,7 @@
 #include "esphome/core/entity_base.h"
 #include "esphome/core/helpers.h"
 
-namespace esphome {
-namespace button {
+namespace esphome::button {
 
 class Button;
 void log_button(const char *tag, const char *prefix, const char *type, Button *obj);
@@ -45,5 +44,4 @@ class Button : public EntityBase, public EntityBase_DeviceClass {
   CallbackManager<void()> press_callback_{};
 };
 
-}  // namespace button
-}  // namespace esphome
+}  // namespace esphome::button