Preserve quoted newlines following backslashes

[repzakske]

Formatting a quoted value containing a backslash followed by LF can remove whitespace and normalize line endings inside that value:

query = "SELECT 'a\\\nb  \r\nc';"
assert sqlparse.format(query) == query

The assertion currently fails: the two spaces after b disappear and its CRLF becomes LF. SPLIT_REGEX uses \\. for quoted escapes, but . does not match LF. Enabling DOTALL keeps the complete quoted value together so the serializer only strips whitespace outside it.

Added 30 regression cases for single/double quotes, LF/CR/CRLF, paired backslashes, and default/whitespace/reindent formatting. Eight fail before the fix. Independent SQLite checks preserve all 36 tested values after the fix; the original formatter changes eight.

• ran the tests (pytest): 536 passed on each of Python 3.12, 3.13 and 3.14; the baseline's two xfails and one xpass are unchanged
• all style issues addressed (ruff check sqlparse/)
• changes are covered by tests
• changes are documented in the changelog