Show underscore `_` directories inside ROM paths only

[higorhgon]

Feature request

Allow directories whose names start with _ to be shown ONLY when browsing inside the configured ROM paths (romPath / secondary romPath) and their subdirectories.

Current behavior

In the file browser (DSi theme), directories with names starting with _ are always filtered out (e.g. _Homebrew, _Pokemon), even when "Show hidden" is enabled. This makes it impossible to use _ as a prefix to keep category folders sorted first.

Why this is useful

I organize categories using _ so they sort first alphabetically, similar to MiSTer FPGA conventions:

• sd:/roms/nds/_Homebrew
• sd:/roms/nds/_Pokemon
• sd:/roms/nds/_Apps
• sd:/roms/gba/_Pokemon

Proposed behavior

• Outside the configured ROM paths: keep hiding _ directories (current behavior)
• Inside ms().romPath[0] and ms().romPath[1] (and any subdirectories): allow _ directories to be listed

This should cover both flashcard (fat:/...) setups (DS Lite) and SD (sd:/...) setups (3DS).

Test setup

• Primary: Nintendo DS Lite + flashcard (fat:/...)
• Secondary: Nintendo 3DS + SD (sd:/...)

Notes

No special sorting change is requested; allowing _ directories should naturally place them before most regular names in the alphabetical ordering.