✨ Support Flutter 2 & Null Safety

[istornz]

• Support Flutter 2.
• Add Null Safety support.
• Update iOS example to work with the latest SDK.

[narcodico]

@phucgaoxam you mind migrating this package to null-safety ? Appreciate it!

[Rileyjrjohns]

@phucgaoxam Bump, please consider this migration :)

[Rileyjrjohns]

@istornz I forked this package with a brand new one, i migrated it to null safety as well. @phucgaoxam Let me know if you update this one, many thanks for your wonderfull initial work ! https://pub.dev/packages/switcher_xlive