Make math rendering opt-in with Trait

[ajevans99]

Summary

• Add a disabled-by-default SwiftPM Math trait
• Include SwiftUIMath only when Math is enabled
• Conditionally compile math parsing, rendering, configuration, and tests
• Preserve standard Markdown and code-block rendering without math support
• Update documentation and configure the demo to opt in

Motivation

SwiftUIMath bundles roughly 7.4 MB of font resources. Applications that do not render math currently inherit this cost unnecessarily.

This change makes math support explicitly opt-in while leaving the rest of Textual’s Markdown functionality unchanged.

Usage

.package(
  url: "https://github.com/gonzalezreal/textual",
  from: "0.5.0",
  traits: [.defaults, "Math"]
)