Pptreport

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- Fixed bug where missing files in `grouped_content` raised an error even when missing_file="text". Combinations of empty_slide=="skip" and missing_file="text"/"empty"/"skip" now work the same as for regular content lists.

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- Reworked markdown parsing to allow for headers (` header`), lists (`- item 1`) and nested types (`**partly bold and _italics_ string**`). Unsupported markdown types are logged as warnings and added as plain text.
- Content patterns with invalid regexes, e.g. "**text**" no longer raise an error, but are now logged as warnings and added as text (which may or may not contain markdown).
- Added restriction of pillow<10 due to python-pptx deprecation: "DeprecationWarning: getsize is deprecated and will be removed in Pillow 10 (2023-07-01). Use getbbox or getlength instead."

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- Initial release