FileSystem: Don't follow symbolic links when recursively deleting directories

[chaoticgd]

Description of Changes

• The FileSystem::RecursiveDeleteDirectory function will no longer recurse through symbolic links.
• To support this change, the FileSystem::IsSymbolicLink and FileSystem::DeleteSymbolicLink functions have been added. The FileSystem::IsRealDirectory function has been removed since it was unused.

Rationale behind Changes

Prevent accidental data loss e.g. when deleting folder memory cards.

Suggested Testing Steps

This needs testing on macOS and especially Windows, since it touches Windows API code and I've only tested it on Linux. Since this is modifying some fairly important code I want multiple core contributors to review it before it's merged.

Here's an example of how you could reproduce the issue on Linux/macOS:

cd path/to/memcards/directory/
pcsx2-qt # create a folder card called "testcard" from the gui
mkdir important_dir
nano important_dir/important_file.txt # add only copy of private key for 1000000 bitcoin reserve
ln -s $(realpath important_dir) testcard.ps2/link # it's important to use an absolute path here
pcsx2-qt # try to delete "testcard" from the gui
ls important_dir/important_file.txt # oh no!

Be careful that you don't e.g. symlink your home directory by accident while testing this, as you could lose all your data.

[chaoticgd]

Updated the test so it always creates a fresh directory.