Closes#1398
### What problem does this PR solve?
Adds native support for EPUB files. EPUB content is extracted in spine
(reading) order and parsed using the existing HTML parser. No new
dependencies required.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
To check this parser manually:
```python
uv run --python 3.12 python -c "
from deepdoc.parser import EpubParser
with open('$HOME/some_epub_book.epub', 'rb') as f:
data = f.read()
sections = EpubParser()(None, binary=data, chunk_token_num=512)
print(f'Got {len(sections)} sections')
for i, s in enumerate(sections[:5]):
print(f'\n--- Section {i} ---')
print(s[:200])
"
```
