chore: split LSP completion.rs into several files (noir-lang#5723)

# Description

## Problem

No issue, it's just that `completion.rs` was already too big.

## Summary

This PR just moves things around without changing any behavior.

## Additional Context

## Documentation

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.