Skip BOM when determining Locator's line starts (#6159)

## Summary

If a file has a BOM, the import sorter _always_ reports the imports as
unsorted. The acute issue is that we detect that the line has leading
content (before the imports), which we always consider a violation.
Rather than fixing that one site, this PR instead makes `.line_start`
BOM-aware.

Fixes https://github.com/astral-sh/ruff/issues/6155.

crates/ruff/resources/test/fixtures/isort/bom_sorted.py

@@ -0,0 +1,2 @@
+import bar
+import foo

crates/ruff/resources/test/fixtures/isort/bom_unsorted.py

@@ -0,0 +1,2 @@
+import foo
+import bar

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

@@ -314,6 +314,8 @@ mod tests {
 
     #[test_case(Path::new("add_newline_before_comments.py"))]
     #[test_case(Path::new("as_imports_comments.py"))]
+    #[test_case(Path::new("bom_sorted.py"))]
+    #[test_case(Path::new("bom_unsorted.py"))]
     #[test_case(Path::new("combine_as_imports.py"))]
     #[test_case(Path::new("combine_import_from.py"))]
     #[test_case(Path::new("comments.py"))]

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

@@ -0,0 +1,4 @@
+---
+source: crates/ruff/src/rules/isort/mod.rs
+---
+

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

@@ -0,0 +1,18 @@
+---
+source: crates/ruff/src/rules/isort/mod.rs
+---
+bom_unsorted.py:1:1: I001 [*] Import block is un-sorted or un-formatted
+  |
+1 |   import foo
+  |  _^
+2 | | import bar
+  |
+  = help: Organize imports
+
+ℹ Fix
+1   |-import foo
+2   |-import bar
+  1 |+import bar
+  2 |+import foo
+
+

crates/ruff_source_file/src/locator.rs

@@ -76,7 +76,11 @@ impl<'a> Locator<'a> {
         if let Some(index) = memrchr2(b'\n', b'\r', bytes) {
             // SAFETY: Safe because `index < offset`
             TextSize::try_from(index).unwrap().add(TextSize::from(1))
+        } else if self.contents.starts_with('\u{feff}') {
+            // Skip the BOM.
+            '\u{feff}'.text_len()
         } else {
+            // Start of file.
             TextSize::default()
         }
     }