[examples] Fix directory navigation in core_directory_files

[MAXDVVV]

Summary

Fixes #5788.

This restores directory navigation in the core_directory_files example after the switch to GuiListViewEx.

What changed

• Include directories in the filtered listing by using DIRS*;.png;.c.
• Detect when the selected list item is a directory and reload the view for that path.
• Reset GuiListViewEx state after forward/back navigation.

Rationale

LoadDirectoryFilesEx(directory, ".png;.c", false) filters out directories, so the list no longer contains entries that can be selected to navigate into subdirectories. Including DIRS* restores directory rows while keeping the existing .png/.c file filtering.

Validation

• Checked LoadDirectoryFilesEx() / GetDirectoryFileCountEx() / ScanDirectoryFiles() filter behavior.
• git diff --check passes.
• No local C toolchain was available in this Windows environment (cmake, gcc, cl not found), so this PR is validated by static review only.

[raysan5]

@MAXDVVV thanks, the fix is valid