feat: support .hidden file for dotfiles exclusion

[NecRaul]

Add support for a per-directory .hidden file to exclude additional files/directories from listings.

• looks for a .hidden file in each directory being listed
• reads filenames from .hidden if it exists
• excludes any files or folders listed in .hidden from listings
• still hides normal dotfiles as before
• if no .hidden file exists, behavior is unchanged

Example directory:

/dir
├─ subdir/
│  ├─ lost+found/
│  └─ file2.txt
├─ lost+found/
├─ .secret
├─ .hidden  (contains "hidden.txt" and "lost+found")
├─ file1.txt
├─ hidden.txt
└─ lost+found.txt

Resulting listing:

/dir
├─ subdir/
│  ├─ lost+found/  (not hidden)
│  └─ file2.txt
├─ file1.txt
└─ lost+found.txt

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This PR complies with the DCO; https://developercertificate.org/