opt in to default prompts and streamline user prompts

[tjex]

Addresses David-Kunz#64.

This PR became more opinionated that intended... I hope it fits!

• Added a boolean option in setup() to include default prompts or not.
• Loading default prompts is now a function call. This gives space for using the prompts.lua file for other uses in the future (eg, other prompt functions or to load prompts based on filtering / specific models etc).
• Custom prompts can be added to the variable user_prompts instead of appended to the default prompts file directly

I'm however starting to pull some hair out... I don't know why the plugin isn't respecting my settings for user_prompts and load_default_prompts. Setting them in my config has no effect. Only their default settings within init.lua are being respected...

Any pointers on that would be highly appreciated (with or without a PR approval).

[tjex]

totally overcomplicated solution, as discussed in respective issue...