fix: Change YOLO import default image extension choices to .jpg, .jpeg, .png

[EdjeElectronics]

This simple PR fixes the default image extension choices when using label-studio-converter to import YOLO data.

• In yolo.py, the function definition for convert_yolo_to_ls uses image_ext=".jpg,.jpeg,.png" as the default image extension options. However, this default choice is overridden by the default --image-ext argument, which is just ".jpg".
• The PR changes the default choices in the --image-ext argument to match the function default, because it seems like that's what was intended by the original authors of the code.
• It changes the default --image-ext argument from ".jpg" to ".jpg,.jpeg.png"

Ultimately, this change makes it easier for a new user to use label-studio-converter without having to know about the --image-ext argument, which isn't documented anywhere. The Tutorial: Importing Local YOLO Pre-Annotated Images to Label Studio doesn't discuss the --image_ext argument at all, so new users could potentially be tripped up when they try to import images that aren't .jpg.