[BUG] PDFs are not re-extracted after they're updated

[scambier]

Problem description:

https://github.com/scambier/obsidian-omnisearch/discussions/291#discussioncomment-7032086

Your environment:

• Plugin version:
• Obsidian version:
• Operating system:
• Number of images/PDFs in your vault (approx.):
• Other plugins that may be related to the issue:

[scambier]

• A PDF is only extracted on demand (e.g. when Omnisearch wants to index it, or when using the contextual menu)
• Once Omnisearch has built its cache, it won't re-extract (or hit the cache of) a PDF unless it appears in a result

A solution would be to add a "automatically extract" opt-in setting in Text Extractor, that would keep the cache up-to-date

[demig00d]

@scambier, what do you think about adding the modification time to the cache file name?

While it may result in a larger number of cache files, it seems like a simpler solution.