Verify an IModel as a Mermaid diagram in markdown, svg or png

[SimonCropp]

Adds support for verifying an EF Core IModel as a Mermaid ER diagram, using DbToMermaid.

await Verify(context.Model)
    .ModelAsDiagram(
        ModelDiagramFormat.Markdown |
        ModelDiagramFormat.Svg |
        ModelDiagramFormat.Png);

• Verify(model) on an IModel now produces the markdown diagram by default.
• ModelAsDiagram selects markdown, svg, png, or any combination. One file is verified per format.
• Adds a dependency on EfToMermaid, which brings in Naiad for svg and png rendering. Naiad's sponsorship build check flows to consuming projects.
• Adds the Papyrine OpenSource exemption to Directory.Build.props.
• Excludes *.verified.md from MarkdownSnippets processing, since it was rewriting the markdown snapshots.
• Bumps the version to 16.1.0.