Fixes the reg class key order for FS items. The existing code was close,
but for some reason used `//` as the path separator for SystemFileAssociations!
- Fixed SystemFileAssociations.
- Swapped the order of `*` and `AllFilesystemObjects`. This is the documented
order and can also be observed in Process Monitor.
https://learn.microsoft.com/en-us/windows/win32/shell/fa-associationarray#about-association-arrays
- Removed `(..., L"%s//%s", extension, wszClass)`, this does not seem to be
a valid thing (`.TestAAExtWeird` in my tests).
- Adds the `Unknown` class when appropriate. Not adding the `openas` verb
to `Unknown` rgs registration now to mimic Windows, because ROS
`CDefaultContextMenu` lacks verb de-duplication and the menu would end up
with two "Open With" entries. This just uses `(cidl == 1)` to simulate
Windows, while Windows on NT6 uses `MultiSelectModel=Single`, a NT6 feature
not implemented in ROS.
- The class order for folders was wrong and is still "wrong" in this PR,
but I chose to use the Windows menu display order until the exact mechanics
required in `CDefaultContextMenu` can be understood.
- Extracts the extension from ANSI PIDLs.
