Add option for code style to optionally include in prompts

[JamesHWade]

Many R programmers prefer a particular style of code (e.g., tidyverse, base-R). It would be nice to adjust the prompts to keep to the programmers preferred style of code writing. This could be done with the option gpttools.code_style.

[JamesHWade]

Forgot to mark off when we implemented it, but it's there in settings of the Chat with Retrieval app.