feat: support custom markdown parsers (#835)

* feat: support custom markdown parsers

* fix: markdown test

* feat: add markdown quote parser

* chore: use markdown parser

* chore: parse unordered list, to_do list and quote

* feat: add a markdown editor example

* feat: add bulleted list parser

* feat: support nested list and img

* feat: support code block

* feat: support table parser

* chore: refacotr code

* fix: flutter analyze

* chore: add more tests and remove unused files

* fix: test

example/lib/pages/markdown/markdown_code_block_parser.dart

@@ -0,0 +1,51 @@
+import 'package:appflowy_editor/appflowy_editor.dart';
+import 'package:appflowy_editor_plugins/appflowy_editor_plugins.dart';
+import 'package:markdown/markdown.dart' as md;
+
+class MarkdownCodeBlockParserV2 extends CustomMarkdownParser {
+  const MarkdownCodeBlockParserV2();
+
+  @override
+  List<Node> transform(
+    md.Node element,
+    List<CustomMarkdownParser> parsers,
+    MarkdownListType listType,
+  ) {
+    if (element is! md.Element) {
+      return [];
+    }
+
+    if (element.tag != 'pre') {
+      return [];
+    }
+
+    final ec = element.children;
+    if (ec == null || ec.isEmpty) {
+      return [];
+    }
+
+    final code = ec.first;
+    if (code is! md.Element || code.tag != 'code') {
+      return [];
+    }
+
+    String? language;
+    if (code.attributes.containsKey('class')) {
+      final classes = code.attributes['class']!.split(' ');
+      final languageClass = classes.firstWhere(
+        (c) => c.startsWith('language-'),
+        orElse: () => '',
+      );
+      language = languageClass.substring('language-'.length);
+    }
+
+    final deltaDecoder = DeltaMarkdownDecoder();
+
+    return [
+      codeBlockNode(
+        language: language,
+        delta: deltaDecoder.convertNodes(code.children),
+      ),
+    ];
+  }
+}