Adds multi-file support to IDE support (#8857)

# Description

This adds multi-file support to the in-progress IDE support. The main
new features are a `-I` flag that allows you to add a new source search
path when starting up the nu binary, and fixes for the current IDE
support to support spans in other files.

This needs accompanying fixes to the vscode/lsp implementation to pass
along the project directory via `-I`.

UPDATE: Marking this draft until we have a means to test this.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-parser/src/parse_keywords.rs

@@ -1719,7 +1719,8 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                 };
 
                 if let Ok(contents) = std::fs::read(&module_path) {
-                    let file_id = working_set.add_file(module_filename, &contents);
+                    let file_id =
+                        working_set.add_file(module_path.to_string_lossy().to_string(), &contents);
                     let new_span = working_set.get_span_for_file(file_id);
 
                     // Change the currently parsed directory

crates/nu-parser/src/parser.rs

@@ -5813,7 +5813,12 @@ pub fn parse(
     scoped: bool,
 ) -> Block {
     let name = match fname {
-        Some(fname) => fname.to_string(),
+        Some(fname) => {
+            // use the canonical name for this filename
+            nu_path::expand_to_real_path(fname)
+                .to_string_lossy()
+                .to_string()
+        }
         None => "source".to_string(),
     };