ftguess - add HWP detection

[decalage2]

add detection for HWP document format, based on OLE.
See https://github.com/volexity/hwp-extract, especially the _is_valid method: https://github.com/volexity/hwp-extract/blob/19361beb4d553cb33a1342e885e86b2d66324916/src/hwp_extract/hwp.py#L132