Extract shared parsing utilities into isort/_parse_utils.py

Move `_infer_line_separator`, `normalize_line`, `strip_syntax`, and `skip_line`
out of parse.py into a new dedicated module (_parse_utils.py) that can be shared
by both parse.py and identify.py without either depending on the other.

parse.py re-imports and re-exports all four names for backward compatibility.
identify.py now imports directly from _parse_utils instead of parse.

Co-authored-by: DanielNoord <13665637+DanielNoord@users.noreply.github.com>