Introduce lightweight style guide

[netrome]

Background

I have a lot of style opinions which most seem to agree makes the code a bit nicer, but I don't want to pollute reviews or repeat these time and time again in comments. Therefore it would be nice to maintain a STYLE.md file with some basics like:

• Preserve namespacing for types/functions/macros with common names, e.g. thiserror::Error > Error, anyhow::Error > Error, tracing::info!() > info!() etc.
• Always preserve namespaces for functions.
• Organize types/functions after occurrence (e.g. if A references B, A should be declared before B).

User Story

As a developer I like well-defined style guides to make the code easier to read and understand.

Acceptance Criteria

• We've added a STYLE.md file with basic style guides.
• LLMs are prompted to follow these.

Resources & Additional Notes

This is a small style guide I've used for a personal side project which has been pretty useful: https://github.com/netrome/magent/blob/main/docs/STYLE.md