Add Mermaid CFG Visualizer alongside DOT implementation

[zyf265600]

Background

The Checker Framework currently supports control-flow graph (CFG) visualization through the DOTCFGVisualizer, which outputs graphs in Graphviz DOT format .
While DOT is powerful, its integration with documentation platforms is limited: GitHub/GitLab Markdown does not natively render DOT, requiring additional tooling or screenshots.

Goal

Introduce a new MermaidCFGVisualizer that outputs CFGs in Mermaid syntax. Mermaid diagrams can be rendered natively in GitHub, GitLab, and many Markdown viewers, making CFGs more accessible in issues, pull requests, and documentation.