Implement AlertBlockSyntax using BlockquoteSyntax

As I understand the alert-block syntax, it is an extension on the
orignal blockquote support in markdown. Basically, it's a blockquote
that is rendered differently.

That means that if alert-block syntax isn't supported a markdown parser
will render it as a blockquote. This provides a nice graceful fallback.
It also means that if you're implementing it, you really should
implement it the same way you implement blockquote parsing.

Author: jonasfj

lib/markdown.dart

@@ -42,7 +42,6 @@ import 'src/version.dart';
 
 export 'src/ast.dart';
 export 'src/block_parser.dart';
-export 'src/block_syntaxes/alert_block_syntax.dart';
 export 'src/block_syntaxes/block_syntax.dart';
 export 'src/block_syntaxes/blockquote_syntax.dart';
 export 'src/block_syntaxes/code_block_syntax.dart';

lib/src/block_syntaxes/alert_block_syntax.dart

@@ -97,3 +97,65 @@ class BlockquoteSyntax extends BlockSyntax {
     return Element('blockquote', children);
   }
 }
+
+/// Parses GitHub Alerts blocks.
+///
+/// See also: https://docs.github.com/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts
+class AlertBlockSyntax extends BlockSyntax {
+  @override
+  RegExp get pattern => alertPattern;
+
+  const AlertBlockSyntax();
+
+  @override
+  List<Line> parseChildLines(BlockParser parser) {
+    return const BlockquoteSyntax().parseChildLines(parser);
+  }
+
+  @override
+  Node parse(BlockParser parser) {
+    // Parse the alert type from the first line.
+    final type =
+        pattern.firstMatch(parser.current.content)!.group(1)!.toLowerCase();
+
+    final childLines = parseChildLines(parser);
+    // Until we've parse all the child lines, we can't actually know if this is
+    // a blockquote containing `[!note]` or if this is an alert-block.
+    //
+    // This is because `> [!note]` is not a valid alert-block!
+    final isBlockquote = childLines.length <= 1;
+
+    if (!isBlockquote) {
+      // Always remove the first line, this is the line that contained the type.
+      childLines.removeAt(0);
+    }
+
+    // Recursively parse the contents of the blockquote.
+    final children = BlockParser(childLines, parser.document).parseLines(
+      // The setext heading underline cannot be a lazy continuation line in a
+      // block quote.
+      // https://spec.commonmark.org/0.30/#example-93
+      disabledSetextHeading: BlockquoteSyntax._lazyContinuation,
+      parentSyntax: this,
+    );
+
+    if (isBlockquote) {
+      return Element('blockquote', children);
+    }
+
+    // Mapping the alert title text.
+    const typeTextMap = {
+      'note': 'Note',
+      'tip': 'Tip',
+      'important': 'Important',
+      'caution': 'Caution',
+      'warning': 'Warning',
+    };
+    final titleText = typeTextMap[type]!;
+    final titleElement = Element('p', [Text(titleText)])
+      ..attributes['class'] = 'markdown-alert-title';
+    final elementClass = 'markdown-alert markdown-alert-$type';
+    return Element('div', [titleElement, ...children])
+      ..attributes['class'] = elementClass;
+  }
+}

lib/src/block_syntaxes/blockquote_syntax.dart

@@ -1,5 +1,5 @@
-import 'block_syntaxes/alert_block_syntax.dart';
 import 'block_syntaxes/block_syntax.dart';
+import 'block_syntaxes/blockquote_syntax.dart';
 import 'block_syntaxes/fenced_code_block_syntax.dart';
 import 'block_syntaxes/footnote_def_syntax.dart';
 import 'block_syntaxes/header_with_id_syntax.dart';

lib/src/extension_set.dart

@@ -129,3 +129,12 @@ Additional markdown text.
 with two lines.</p>
 </div>
 <p>Additional markdown text.</p>
+>>> #584
+> [!CAUTION]
+>
+> some warning
+<<<
+<div class="markdown-alert markdown-alert-caution">
+<p class="markdown-alert-title">Caution</p>
+<p>some warning</p>
+</div>