Add `FlxGroup::revive()`

The FlxGroup will now revive itself, and then all its members.

This is to compliment `FlxGroup::kill()`.
Fix minor typos in `FlxGroup::revive()`

It's a minor typo, but with major consequences!

If left unchecked, this bug would revive already existing objects,
leading to several Science Fiction worthy paradoxes.

Also fixed a tiny spelling typo in the ASDoc.
Only revive `FlxGroup` objects if they are `alive`

After some discussion on GitHub
https://github.com/FlixelCommunity/flixel/pull/110/files#r1831839
it seems the more logical to check if an object is `alive`,
rather than check if it `exists` when reviving it.
Merge pull request #110 from IQAndreas/fix-flxgroup-kill

Add `FlxGroup::revive()`