Include the ShellLink plugin in NSIS

The Mullvad Browser installer will include an uninstaller.

To do things properly, we could check whether the desktop and start menu .lnk points to the installation we're deleting.

This requires an additional plugin for NSIS.

It's just a .cpp file, but it needs a small patch to fix an #include. We could just keep the file in projects/nsis, rather than downloading the full .zip just for that file.

For some Python scripts we already to this, e.g., the ones to extract macOS SDK, but we keep the originals and a patch to adapt them to our build system. However, one reason to keep the patch is to make it easy to adapt new releases, whereas I don't think this plugin will be updated again.