Add a quick format option for mkfat32

Added open implementation

Needed to return a pointer to the file system as `@fieldParentPtr(Fat32Self, "instance", fs.instance)` doesn't work as the the pointer to `instance` is on the stack, then goes out of scope when the init is returned. So will be using free stack space.
Part of #216

Spelling, doc comments and extra test for open file


init -> create, deinit -> destroy

src/kernel/filesystem/vfs.zig

@@ -245,6 +245,9 @@ pub const Error = error{
 
     /// No symlink target was provided when one was expected
     NoSymlinkTarget,
+
+    /// An unexpected error ocurred when performing a VFS operation.
+    Unexpected,
 };
 
 /// Errors that can be thrown when attempting to mount