Introduce Defaults package into project and implement Setting - General with SwiftUI

[CanglongCl]

close #287

[github-actions]

Hello CanglongCl, Thank you for your first PR contribution 🎉 CanglongCl

[CanglongCl]

I'm not sure how the project store other type in original storage. To use Defaults with other type like EZLanguage, we need to implement Defaults.Serializable for them. What's our target to refactor them? Write a new one in Swift or maintain objc for a while?

[tisfeng]

EZLanguage can be rewritten with Swift, however I am not sure if this would affect the objc code that use EZLanguage, as it is used in many places.

If other objc code will not be affected, then EZLanguage can be rewritten directly. If there is an impact, many places will have to make corresponding changes, which may be a lot of work.

Overall, I am in favor of rewriting EZLanguage with Swift.

[CanglongCl]

BTW I think this PR can be just merged. Other related migration can be based on Defaults package.

[CanglongCl]

GeneralTab is almost completed except keyboard shortcut section. I've also added the beta feature toggle as mentioned in #321 and #135.

[tisfeng]

That’s great, it works well.

Just two more options are missing: appearance and font size, could you add those please?

[CanglongCl]