[modbus] Split modbus_server from modbus_controller (#15509)

Co-authored-by: J. Nick Koston <nick+github@koston.org>

CODEOWNERS

@@ -347,6 +347,7 @@ esphome/components/modbus_controller/select/* @martgras @stegm
 esphome/components/modbus_controller/sensor/* @martgras
 esphome/components/modbus_controller/switch/* @martgras
 esphome/components/modbus_controller/text_sensor/* @martgras
+esphome/components/modbus_server/* @exciton
 esphome/components/mopeka_ble/* @Fabian-Schmidt @spbrogan
 esphome/components/mopeka_pro_check/* @spbrogan
 esphome/components/mopeka_std_check/* @Fabian-Schmidt