Zero Install for Windows 2.18 released

Zero Install for Windows 2.18 has been released. This improves context menu
desktop integration (icons, cascading menus, directory background menus) and
archive extraction (support for .tar.lz, .tar.zst and .rar).

Download: https://get.0install.net/#windows

## Changes

### Desktop integration

- Added context menu icons
- Added support for cascading context menus (see
https://docs.0install.net/specifications/capabilities/#context-menu-entries)
- Added entries to directory background menus
- Added support for <arg>s in <verb>s (see
https://docs.0install.net/specifications/capabilities/#verbs)
- Improved icon caching

### Archive extraction

- Added support for extracting `.tar.lz`, `.tar.zst` and `.rar` archives
- Improved performance when extracting `.tar.lzma`, `.tar.xz` and `.7z`
archives

### Misc

- Improved support for Sync with local/network filesystem (see
https://docs.0install.net/details/sync/)
- Added support for detecting .NET Framework 4.8