feat: add beautiful-mermaid as optional mermaid renderer

[dermorz]

Summary

Adds support for beautiful-mermaid as an alternative to the default mermaid.js renderer for rendering diagrams in Markdown and AsciiDoc files.

Changes

• Add mermaid.renderer config option ("default" or "beautiful")
• Add mermaid.theme config option for beautiful-mermaid themes
• Bundle beautiful-mermaid library for offline use
• Update HTML template to conditionally load the selected renderer
• Update client-side JavaScript to use the configured renderer
• Add documentation in README and help docs

Usage

require('livepreview.config').set({
    mermaid = {
        renderer = "beautiful",
        theme = "github-light",
    }
})

Available beautiful-mermaid themes: zinc-light, zinc-dark, tokyo-night, tokyo-night-storm, tokyo-night-light, catppuccin-mocha, catppuccin-latte, nord, nord-light, dracula, github-light, github-dark, solarized-light, solarized-dark, one-dark.

Note

beautiful-mermaid supports 6 diagram types (flowchart, state, sequence, class, ER, xychart). Some mermaid.js diagrams may not render with this renderer.

Breaking Changes

None - defaults to existing mermaid.js renderer.

[github-actions]

Thank you for your pull request! We appreciate your contribution. Please ensure that your changes are well documented and tested, and are formatted as noexpandtab.