Changes:
- New clang64 environment
- Installation or first startup after unpacking will no longer leave idle background processes around
- Installer:
- "maintenancetool.exe" was renamed to "uninstall.exe" and its features reduced to just uninstallation. See #25
- documentation added for silent installation: https://github.com/msys2/msys2-installer/tree/2021-06-04#cli-usage-examples