CustomButton

G'day @sindresorhus! Thanks so much for all of your open source work. I recently started learning how to write macOS apps from scratch and your code has been extremely valuable to study! 😁 For your consideration, this PR proposes adding another property to this button class that enables setting the type of corner rounding curve to use Apple's classic "squircle" style that was introduced as a public API in macOS 10.15. Seeing as this is now the default curve style in Big Sur, I've set this property to by default, but I'm happy to change it if this is considered a breaking change. Please let me know if this PR is good as-is, or if you need me to change anything (Or if you don't need it at all! XD) Thanks! Have a great day!

I have ensured all button properties are set during setup and only animated properties are set/changed during animation. This prevents the animation of the initial color as described in your issue. Fixes #1