escape or reject a bare carriage return in multiline strings

[netliomax25-code]

Summary

1. String.from_raw builds a multiline basic string by escaping every control character except a line feed and a carriage return, so a lone CR (one that is not the CR of a CRLF pair) is written out raw.
2. The multiline literal path drops that same CR from invalid_sequences, so a lone CR there is neither escaped nor rejected.
3. Either way tomlkit.string(value, multiline=True) and its literal variant emit a document with a raw lone CR, which the tomlkit parser and the stdlib tomllib both reject as a control character, so the value stops round-tripping.
4. Escape a lone CR as \r for multiline basic strings, and raise InvalidStringError for multiline literal strings; LF and CRLF line endings are left untouched.

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