Fix parsing and unparsing ascii files with escaped characters and NA strings

[trossi]

Describe the proposed changes

The PR will

• fix parsing and unparsing ascii files with strings including e.g. '?' and some other characters
• fix unparsing ascii files with NA strings
• add test files for the corresponding cases (R code used to generate the test files added in the comment of tests)

Additional information

A test file is added in xdr format too to check that there is no issues there (there was none).

Checklist before requesting a review

• I have performed a self-review of my code
• The code conforms to the style used in this package (checked with Ruff)
• The code is fully documented and typed (type-checked with Mypy)
• I have added thorough tests for the new/changed functionality

[trossi]

I pushed a fix for unparsing NA strings too and updated the PR description accordingly.

[vnmabus]

Just one comment/question. The rest seems fine.

[vnmabus]

LGTM