SDXL Style JSON Enhancement

[Lech Deregowski]

I am not certain if this would be beneficial to some or all, however it would be nice if the single sdxl_styles.json file could be broken up into their individual/constituent style files for ease of use/expansion versus editing the single file by hand. This would curb the need for pull requests when new common styles are discovered, Alternately to look for a "user_sdxl_styles.json" file to allow custom styles to be stored that could persist between updates would be a great feature.