feat: new image_quality and sixel_fraction options to allow users to configure the image preview quality

[sxyazi]

Closes #446 - @baitian752
Closes #562 - @AlexKurisu

• image_quality: Quality on pre-caching images, range 50-90. The larger value, the better image quality, but slower with more CPU consumption, and generates larger cache files that occupy more storage space.
• sixel_fraction: Sixel is a very old image format that only supports 256 colors. For better image preview, Yazi trains a neural network for each image to find the most representative colors. This value determines the number of samples used during the training, range 10-20. A smaller value produces better results but is also slower.

See the docs here https://yazi-rs.github.io/docs/configuration/yazi#preview