Convenient function to init type checker

[katat]

so that it can simplify and encapsulate the following construction:

let mut sources = Sources::new();
let mut tast = TypeChecker::new();
let this_module = None;
let _node_id = typecheck_next_file(
    &mut tast,
    this_module,
    &mut sources,
    file_name.to_string(),
    code.clone(),
    0,
)
.unwrap();