Heat a heavy-bottomed skillet over medium-high heat. Then, add the butter and brown the chicken for 1 minute on each side. The chicken will be browned but still raw. Next is to remove to a plate. In the same skillet, cook the diced onion and celery for about 4 minutes until softened. Add minced garlic and cook for another 2 minutes.
Add everything besides noodles to the slow cooker. Place the browned chicken, cooked onion and celery mixture, chicken broth, salt, pepper, dried basil, dried oregano, and sliced carrots into the slow cooker. Cover and cook on low for 3-4 hours or until the chicken is fully cooked and tender.
Shred meat and add noodles. Remove the chicken breasts from the slow cooker. Discard the skin and bones, and shred or chop the meat into bite-sized pieces. Return the shredded chicken to the slow cooker. Add the egg noodles and cook on low for another 30-60 minutes or until the noodles are tender. (Check every 15 minutes to avoid overcooking.)
Mix and serve. Turn off the slow cooker once the noodles are tender to prevent overcooking. Stir the soup well and serve hot. You can garnish with fresh parsley if you want.