Move line_numbers under 'aiken-lang'

And add some unit tests to ensure things are working properly.

crates/aiken-lsp/src/server/lsp_project.rs

@@ -1,10 +1,8 @@
 use std::{collections::HashMap, path::PathBuf};
 
-use aiken_lang::ast::Tracing;
+use aiken_lang::{ast::Tracing, line_numbers::LineNumbers};
 use aiken_project::{config::Config, error::Error as ProjectError, module::CheckedModule, Project};
 
-use crate::line_numbers::LineNumbers;
-
 #[derive(Debug)]
 pub struct SourceInfo {
     /// The path to the source file from within the project root