✨ Add null safety & flutter 2 support

[istornz]

• Add null safety & flutter 2 support
• Fix missing background image asset
• Update iOS Xcode project

[alnitak]

Hi @istornz and thanks for the commits!
Been a long time I wish to update this package.
I rewrote the code removing the menu setup boilerplate and improved performances using only one animation controller instead of one for each item. It works also on all platforms.
Also, the blur effect, as like the background color, can be enabled/disabled.

[istornz]

@alnitak Hello !
Perfect this is very nice 😊