Fix parsing failure for unindented doctest comments

[Erotemic]

Summary

• update the doctest parser to infer indentation for comment placeholders so they no longer trigger IndentationError when comments ignore block indentation
• add a regression test that covers doctests using unindented comment separators

Testing

• pytest tests/test_parser.py -k comment

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Codex Task

[Erotemic]

Fixes #184