importer: skip whitespace between comments at start of file (#6523)

## Summary

When adding an import, such as when fixing `I002`, ruff doesn't skip
whitespace between comments, but isort does. See this issue for more
detail: https://github.com/astral-sh/ruff/issues/6504

This change would fix that by skipping whitespace between comments in
`Insertion.start_of_file()`.

## Test Plan

I added a new test, `comments_and_newlines`, to verify this behavior. I
also ran `cargo test` and no existing tests broke. That being said, this
is technically a breaking change, as it's possible that someone was
relying on the previous behavior.

crates/ruff/resources/test/fixtures/isort/required_imports/comments_and_newlines.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python3
+# A copyright notice could go here
+
+# A linter directive could go here
+
+x = 1

crates/ruff/src/importer/insertion.rs

@@ -67,9 +67,13 @@ impl<'a> Insertion<'a> {
             TextSize::default()
         };
 
-        // Skip over commented lines.
+        // Skip over commented lines, with whitespace separation.
         for line in UniversalNewlineIterator::with_offset(locator.after(location), location) {
-            if line.trim_whitespace_start().starts_with('#') {
+            let trimmed_line = line.trim_whitespace_start();
+            if trimmed_line.is_empty() {
+                continue;
+            }
+            if trimmed_line.starts_with('#') {
                 location = line.full_end();
             } else {
                 break;

crates/ruff/src/rules/isort/mod.rs

@@ -762,6 +762,7 @@ mod tests {
     }
 
     #[test_case(Path::new("comment.py"))]
+    #[test_case(Path::new("comments_and_newlines.py"))]
     #[test_case(Path::new("docstring.py"))]
     #[test_case(Path::new("docstring.pyi"))]
     #[test_case(Path::new("docstring_only.py"))]
@@ -791,6 +792,7 @@ mod tests {
     }
 
     #[test_case(Path::new("comment.py"))]
+    #[test_case(Path::new("comments_and_newlines.py"))]
     #[test_case(Path::new("docstring.py"))]
     #[test_case(Path::new("docstring.pyi"))]
     #[test_case(Path::new("docstring_only.py"))]

crates/ruff/src/rules/isort/snapshots/ruff__rules__isort__tests__required_import_comments_and_newlines.py.snap

@@ -0,0 +1,20 @@
+---
+source: crates/ruff/src/rules/isort/mod.rs
+---
+comments_and_newlines.py:1:1: I002 [*] Missing required import: `from __future__ import annotations`
+  |
+1 | #!/usr/bin/env python3
+  |  I002
+2 | # A copyright notice could go here
+  |
+  = help: Insert required import: `from future import annotations`
+
+ℹ Fix
+2 2 | # A copyright notice could go here
+3 3 | 
+4 4 | # A linter directive could go here
+  5 |+from __future__ import annotations
+5 6 | 
+6 7 | x = 1
+
+

crates/ruff/src/rules/isort/snapshots/ruff__rules__isort__tests__required_import_with_alias_comments_and_newlines.py.snap

@@ -0,0 +1,20 @@
+---
+source: crates/ruff/src/rules/isort/mod.rs
+---
+comments_and_newlines.py:1:1: I002 [*] Missing required import: `from __future__ import annotations as _annotations`
+  |
+1 | #!/usr/bin/env python3
+  |  I002
+2 | # A copyright notice could go here
+  |
+  = help: Insert required import: `from future import annotations as _annotations`
+
+ℹ Fix
+2 2 | # A copyright notice could go here
+3 3 | 
+4 4 | # A linter directive could go here
+  5 |+from __future__ import annotations as _annotations
+5 6 | 
+6 7 | x = 1
+
+