Fix Bug with Introducers in Successions

[Tau-Laboratory]

Overview: What does this pull request change?

Fix Issue described in #4257 It's currently only handling the case where introducers are added directly to the succession. Adding another animation group wont work

Fixed Bug in Successions causing mobjects to be added before they should have been introduced by an animation

Motivation and Explanation: Why and how do your changes improve the library?

I've extracted a private method _get_unintroduced_mobjects in the succession parent AnimationGroup. I did this to avoid duplicating the code creating the group

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[ImadABID]

@Tau-Laboratory, this solution doesn't work for cases like the following:

class SuccessionEdgeCase(Scene):
    def construct(self):
        triangle = Triangle()

        self.play(
            Succession(
                Wait(1),
                AnimationGroup(FadeIn(triangle)),
                Wait(1),
                AnimationGroup(FadeOut(triangle))
            )

Please check this comment.