Supporting SF Symbols identifiers for script icons

[nd-net]

Just a small change that also supports SF Symbols identifiers as script icon names:

• For compatibility purposes, images in the Asset catalog take precedence.
• If an appropriate image hasn't been found there, Boop tries to get the SF Symbol using NSImage(systemSymbolName:accessibilityDescription:) (only on macOS 11+).
• If a script specifies an icon but nothing with the appropriate name can be found, then the icon is nil
• If a script does not specify an icon, then the icon defaults to icons8-unknown, just as it does previously

I did not change the .lowercased() in the initialisation of Script since SF Symbols also uses lowercase-only identifiers.

The documentation has been updated appropriately.

Overview of the new functionality

Specifiying an SF Symbol name in a script

Selection

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[nd-net]

The automatic build on bitrise fails because it's using Xcode 11.7; the NSImage(systemSymbolName:accessibilityDescription:) API is only available on macOS 11/Xcode 12 and above.

[IvanMathy]

Well, this is ridiculously cool. Thanks for the PR!

I updated the CI system's Xcode version so it'll build okay now 👍

[grzesimaj-74]

Dlaczego to jest tylko do Apple App Store? Ja chcę do Microsoft Lumia lub Surface. Nigdy mnie nikt nie namówi do przesiadki na Android lub Apple. Mam wiarygodne informacje których nie mogę publicznie udostępnić. Android i Apple działają wspólnie na nie korzyść dla firmy Microsoft.