Add GFM Alert Styling

[Sam Clark (28pins)]

Summary

What changed: Added support for alerts, like [!NOTE]
Why: to give users and LLMs more flexibility and better UX

API change: BlockQuoteType now includes BlockQuoteAlertKind as an optional. See below:

Old:

text(String)
case nested([BlockQuoteType])

New:

case text(String, BlockQuoteAlertKind?)
case nested([BlockQuoteType], BlockQuoteAlertKind?)

Validation

Checked demos and stress-tested kitchen sink demo

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed.
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

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Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Suppressed comments (1)

Sources/MarkdownText/UI/BlockQuoteView.swift:104

• These lines contain tab-indentation, which is likely to trip SwiftLint and makes the new alert view code inconsistent with the surrounding file’s spacing-based indentation.

			Image(systemName: style.imageName)
				.foregroundStyle(style.accentColor)
				.padding(.leading, 8.0)