Add strikethrough support

[aud]

This adds a simple strikethrough LLM processor that is capable of detecting
strikethrough text.

Fyi: I wrote tests, but was getting errors related to pulling the
datalab-to/pdfs dataset when running the LLM processor suite locally.

Here's a test script with a sample PDF:
strikethrough.pdf.

Rendered PDF: https://gist.github.com/aud/2af33313f945a397b28e7ca728a85d8b

from marker.converters.pdf import PdfConverter
from marker.models import create_model_dict
from marker.config.parser import ConfigParser

config = {
    "output_format": "markdown",
    "use_llm": True,
}

config_parser = ConfigParser(config)

converter = PdfConverter(
    config=config_parser.generate_config_dict(),
    artifact_dict=create_model_dict(),
    processor_list=config_parser.get_processors(),
    renderer=config_parser.get_renderer(),
    llm_service=config_parser.get_llm_service()
)

rendered = converter("strikethrough.pdf")
print(rendered.markdown)

[github-actions]

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[aud]

I have read the CLA Document and I hereby sign the CLA

[tarun-menta]

This looks great, thank you for the PR! Will work on integrating into our next release :)

[yilinjuang]

Hi! Just checking in on the status of this PR. Any updates on review or when it might make it into a release? This feature would be really helpful for revision/review-type documents. Thanks!