Apple Silicon support

[nicoverbruggen]

As far as I can tell, PHP Monitor works just fine on macOS Big Sur (on Intel Macs).

I do not have access to a Developer Transition Kit, so I cannot test on arm64, nor do I plan to release an Universal 2 binary before consumer Macs are available (and we know the status of Homebrew, PHP, Valet...).

The documentation should reflect this information.

• Update documentation to include macOS Big Sur support
• Update README to include new icon + new screenshots
• Make a Universal build with Xcode 12.2