Auto-mount archive next to the executable

Summary

Add an InitPhysFSAuto() (or similar) helper that automatically searches for and mounts a default archive without the caller having to figure out the path. This mirrors the convention used by LÖVE, SDL-based games, and several other engines, and removes a chunk of boilerplate from typical raylib game startup code.

Proposed behavior

Given a filename like "data.zip", search in this order and mount the first hit:

1. Next to the running executable (use raylib's GetApplicationDirectory()).
2. The current working directory.
3. A loose folder of the same base name (e.g. data/) for development builds where assets aren't packed yet.

If none are found, return false and let the caller decide what to do.

Proposed API

bool InitPhysFSAuto(const char* archiveName, const char* mountPoint);

• archiveName — e.g. "data.zip". May be NULL to fall back to a default.
• mountPoint — passed through to PhysFS, may be NULL for root.

Why

• Removes the most common ~10 lines of "figure out where my assets live" code from every project that uses this library.
• Makes "ship a single executable + zip" workflows trivial.
• Keeps dev-mode (loose folder) and release-mode (zip) using the same code path.

Notes

• Should respect RAYLIB_PHYSFS_PLATFORM_RAYLIB so it works on platforms where raylib provides its own file I/O.
• Consider also exposing the resolved path via an out-parameter for logging.

Start by tracing the existing PhysFS initialization entry points and the platform-specific path handling controlled by RAYLIB_PHYSFS_PLATFORM_RAYLIB. Verify how raylib's GetApplicationDirectory() and current-directory checks can support the requested search order, then confirm completion with archive, loose-folder, NULL-argument, mount-point, and not-found cases; the issue names no files or tests.