ensure that when nu evaluates files, it allows early returns (#7415)

# Description

Fixes #7301.

# User-Facing Changes

`return` can now be used in scripts without explicit `def main`.

# Tests + Formatting

Don't forget to add tests that cover your changes. (I'm not sure how to
test this.)

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

# 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-command/tests/commands/return_.rs

@@ -23,3 +23,15 @@ fn early_return_if_false() {
 
     assert_eq!(actual.out, r#"100"#);
 }
+
+#[test]
+fn return_works_in_script_without_def_main() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            nu early_return.nu
+        "#
+    ));
+
+    assert!(actual.err.is_empty());
+}

crates/nu-command/tests/commands/source_env.rs

@@ -314,3 +314,15 @@ fn source_env_const_file() {
         assert_eq!(actual.out, "foo");
     })
 }
+
+#[test]
+fn source_respects_early_return() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            source early_return.nu
+        "#
+    ));
+
+    assert!(actual.err.is_empty());
+}