Improve macOS 11 compatibility

[sindresorhus]

Fixes #57

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This cannot be released until Xcode 12 final is out, but if you really need this now, you can use the Git commit in the Xcode's Swift Package Manager tab.

TODO:

• [x] Check if NSImage.preferencesGeneralName becomes the correct SF Symbol before the final macOS 11 release, if so, we could revert to that. Currently, it's colored even on macOS 11. Update: It does not adopt the Big Sur look, so it cannot be used.
• [x] Document how to do backwards compatibility. Export the Big Sur icons and then use them.
• [x] Improve the segmented control style for macOS 11.

[urtti]

Thanks for working on this, currently the toolbarItems style cuts a bit of the text in Big Sur. I suppose you are aware.

To be more specific, I see this with version 1.0.1.

[sindresorhus]

This is what I see on beta 5 with this PR (ignore the missing icons):

[urtti]

This is what I see on beta 5 with this PR (ignore the missing icons):

Yep, that looks better. Without the PR it tries to show it inline with the window controls on the left, resulting in a funky look.