feat: add --prompt-file and --negative-prompt-file flags

[grencez]

as an alternative to passing long strings to the --prompt and --negative-prompt flags.

Summary

What: Provide alternative flags that populate the prompt and negative_prompt variables from file content.
Why: Putting the prompts in files makes them easier to edit and leak less info on a multi-user system.

Granted, there are other ways to accomplish those goals, but I found this to be the best for my use case. I've been maintaining a patch locally for ~2 months now.

Related Issue / Discussion

#1199 was filed by someone else near the start of the year, so I'm probably not the only one who wants this feature.

Additional Information

I tested manually with good cases:

• file exists
• /dev/stdin

I also tested some error cases to make sure it doesn't crash or silently succeed:

• file not found
• no read permission
• broken symlink
• directory instead of file

Checklist

• I have read and confirmed this PR follows the contribution guidelines.