fix: normalize JPG→JPEG format string before passing to Pillow

[TheSkyentist]

Summary

• to_image_file() documents file_type='JPG' as a valid argument, but Pillow's Image.save() only recognises 'JPEG' — passing 'JPG' raises KeyError: 'JPG' at runtime.
• Fix normalises the alias at the call site (_save_list_to_image_file, line 574) with a single conversion before the img.save() call.
• Adds test_to_image_file_jpg to cover the previously-crashing code path.

Reproducer (before fix)

import ascii_magic
from PIL import Image

art = ascii_magic.AsciiArt(Image.open("photo.jpg").convert("RGB"))
art.to_image_file("out.jpg", file_type="JPG")
# KeyError: 'JPG'

Test plan

• test_to_image_file_jpg passes (new test exercising file_type='JPG')
• All existing test_to_image_file_* tests continue to pass (9/9)

🤖 Generated with Claude Code