Add option to manually change `PROMPT_PROCESSING_CHUNK_SIZE` in the GUI

The value of this element is hardcoded to `512` which may be fine for some MacBooks, however for advanced users this one should be available to play with since it can influence prefill speed a bit.

Code reference: https://github.com/lmstudio-ai/mlx-engine/blob/57634aff2fe2b763093624d0cc0bd2e259845b9b/mlx_engine/cache_wrapper.py#L15