Updated maximum size and minimum generation size to accommodate SDXL generation sizes.

[JoshBaldwin101]

Seeing as SDXL models have been out for a while now, I was surprised to see this project doesn't support it's most common generation sizes.

My changes bring the minimum generation size to 512 and the maximum to 2048. I know 2048 is overkill but call it future-proofing. These new slash-command options do not exceed Discord's limit of 25 previewed options. My changes also hit all of the recommended SDXL generation sizes.

Stabililty AI has stated that the following generation sizes are ideal for SDXL models:

• 1024 x 1024
• 1152 x 896
• 896 x 1152
• 1216 x 832
• 832 x 1216
• 1344 x 768
• 1536 x 640
• 640 x 1536

My solution hits all of these and more without changing the 64 increment rule already instated.

[Kilvoctu]

Heya, does this conflict with PR #236 ?

[JoshBaldwin101]

@Kilvoctu yes, it does. I'll close this request and make another with changes that don't conflict.