Add an extension to bring SFSymbols to the SwiftUI Label

[mallman]

Add an extension to bring SFSymbols to the SwiftUI Label.

Please change code formatting to your liking. I will not be offended.

[mallman]

I just noticed that Label will bump up the version requirements for the platforms in your package. If you want, I can modify my PR to update those deps in the package descriptor. Or I can add version requirement macros around the Label extension. LMK.

[ActuallyTaylor]

This all looks really good, thank you a ton! I just updated the formatting and added an availability check so there should be no need to bump the entire package version.