fix(writer): escape the escape byte when double_quote is disabled

[jaideeppyne]

When double_quote is disabled, the writer escapes quote characters with the escape character (default \), but it did not escape occurrences of the escape character itself in field data. Because the reader treats the escape character inside a quoted field as escaping the following byte, any escape character in the data was silently consumed on read-back.

Repro (current master):

let mut wtr = WriterBuilder::new().double_quote(false).from_writer(vec![]);
wtr.write_record([r"C:\path\to", "END"]).unwrap();
let data = wtr.into_inner().unwrap();
let mut rdr = ReaderBuilder::new().has_headers(false)
    .double_quote(false).escape(Some(b'\\')).from_reader(&*data);
// reads back ["C:pathto", "END"] instead of ["C:\path\to", "END"]

A lone \ field is worse: it swallows the closing quote and the following delimiter, mangling the whole record.

The writer already force-quotes fields containing the escape byte (requires_quotes[escape] = true), so the fix is to also double the escape byte inside csv_core::write::quote when double_quote is false, making the escaping self-consistent and the round-trip lossless. The default (double_quote = true) path is unchanged. Adds regression tests at the csv-core and csv levels.