Add Automaton.toMermaid() for emitting mermaid state charts

### Description

We currently have .toDot() but github supports inline mermaid [2] graphs [1], like this:
```mermaid
graph LR;
A-->B;
A-->C;
B-->D;
C-->D;
```
and it'd be a nice to have for copy-pasting into issues that touch automata in Lucene.

[1] https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams#creating-mermaid-diagrams
[2] https://mermaid.js.org/syntax/stateDiagram.html

Start by locating Automaton.toDot() and the related automaton tests or usage sites. Read how the existing DOT output represents states and transitions, then define completion as a toMermaid() method that emits an equivalent Mermaid state chart suitable for copying into GitHub issues.

Written by the indexing model from the issue text.