This changes the highlighting of Go code to make it more similar to how
we highlight Rust

* normal variables have the normal color, vs. being highlighted. This
really stuck out.
* brackets are properly highlighted

It also brings it closer to Neovim's tree-sitter highlighting by
changing how struct fields are highlighted.



Release Notes:

- Improved highlighting of Go code by tuning highlighting of variables,
brackets, and struct fields.

## Before & After

![screenshot-2024-02-02-11 38
08@2x](https://github.com/zed-industries/zed/assets/1185253/a754f166-89c1-40e8-a8da-b63155180896)