chore: fetch_file now returns &str instead of a File

[kevaundray]

Description

This makes the API more uniform because to add a file, we call:

    pub fn add_file_with_source(&mut self, file_name: &Path, source: String) -> Option<FileId> {

and now to fetch a file we call:

    pub fn fetch_file(&self, file_id: FileId) -> &str;

Instead of returning a File which was not present when we were adding a File

Problem*

Resolves

Summary*

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.