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.

src/command.rs

@@ -37,7 +37,9 @@ pub(crate) fn gather_commandline_args() -> (Vec<String>, String, Vec<String>) {
             #[cfg(feature = "plugin")]
             "--plugin-config" => args.next().map(|a| escape_quote_string(&a)),
             "--log-level" | "--log-target" | "--testbin" | "--threads" | "-t"
-            | "--ide-goto-def" | "--ide-hover" | "--ide-complete" => args.next(),
+            | "--include-path" | "-I" | "--ide-goto-def" | "--ide-hover" | "--ide-complete" => {
+                args.next()
+            }
             _ => None,
         };
 
@@ -109,6 +111,7 @@ pub(crate) fn parse_commandline_args(
             let log_level: Option<Expression> = call.get_flag_expr("log-level");
             let log_target: Option<Expression> = call.get_flag_expr("log-target");
             let execute: Option<Expression> = call.get_flag_expr("execute");
+            let include_path: Option<Expression> = call.get_flag_expr("include-path");
             let table_mode: Option<Value> =
                 call.get_flag(engine_state, &mut stack, "table-mode")?;
 
@@ -142,6 +145,7 @@ pub(crate) fn parse_commandline_args(
             let log_level = extract_contents(log_level)?;
             let log_target = extract_contents(log_target)?;
             let execute = extract_contents(execute)?;
+            let include_path = extract_contents(include_path)?;
 
             let help = call.has_flag("help");
 
@@ -183,6 +187,7 @@ pub(crate) fn parse_commandline_args(
                 log_level,
                 log_target,
                 execute,
+                include_path,
                 ide_goto_def,
                 ide_hover,
                 ide_complete,
@@ -220,6 +225,7 @@ pub(crate) struct NushellCliArgs {
     pub(crate) log_target: Option<Spanned<String>>,
     pub(crate) execute: Option<Spanned<String>>,
     pub(crate) table_mode: Option<Value>,
+    pub(crate) include_path: Option<Spanned<String>>,
     pub(crate) ide_goto_def: Option<Value>,
     pub(crate) ide_hover: Option<Value>,
     pub(crate) ide_complete: Option<Value>,
@@ -249,6 +255,12 @@ impl Command for Nu {
                 "run the given commands and then enter an interactive shell",
                 Some('e'),
             )
+            .named(
+                "include-path",
+                SyntaxShape::String,
+                "set the NU_LIB_DIRS for the given script",
+                Some('I'),
+            )
             .switch("interactive", "start as an interactive shell", Some('i'))
             .switch("login", "start as a login shell", Some('l'))
             .named(