📂 Drop folders in pattern expansion

[agoose77]

This PR addresses #1323 by:

1. Simplifying patterns to only generate file entries
2. Drop support for pattern entries with children -- in the schema, pattern entries can only be leaves.

[fwkoch]

Only one critical change (warn about ignored children) - but I'd also love to see some of the tests persist, so the new behavior is clear.

[agoose77]

@fwkoch this function now performs the directory-sensitive string comparison:

1. Paths with non-matching directories are nat-sorted
2. Paths with matching directories are nat-sorted if neither file is an index, or both files are index files
3. Paths with non-matching directories are sorted in preference of the index file if one file is an index

Does this match your expectations?

[fwkoch]

I like this much better. There's still a couple subtle differences between pattern expansion and implicit toc construction, but I don't think they matter? Just wanted to mention them so we are clear that the differences are intentional.