Feature request: Graph theory loops, weighted edges

Hello. I would like to animate several algorithms used in computer science. I've noticed it is not possible to add loop edges (edge from a node to itself) and it is not possible to assign weights to edges (decimal numbers). Can you add this functionality?

Look at the graph theory module in manim, likely under manim/mobject/graph.py or similar. Examine how edges and nodes are currently defined and rendered. Determine how to modify the data structures to support loops and edge weights, and update the rendering logic to display them. Check existing tests for graph functionality to ensure changes don't break existing behavior.

由索引模型根据 Issue 内容生成。