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_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()
         }
     }