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.
