Create LabeledRectangle Class

[Fineas]

Overview: What does this pull request change?

Introduce a new Class in mobject/geometry/polygram.py

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

While working with the Graph class I encountered issues with vertexes of type Rectangle, whenever I set labels=True. Thus, I looked at how LabeledDot is defined in arc.py and implemented a Rectangle with support for Label.

Result:

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

[JasonGrace2282]

To be clear, we didn't forget about this PR. The devs were discussing whether it would be more practical to create a LabeledMixin class, instead of LabeledDot LabeledRectangle and whatever other Labeled* classes people want.