🚸 Rewrite the markdown deserializer to improve br… (#1198)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
- Updated markdown handling and serialization libraries for improved
performance and accuracy in text formatting.
- **New Features**
- Enhanced rich text and markdown conversion capabilities, providing
users with more reliable and seamless text formatting options.
- **Documentation**
- Added detailed documentation for markdown to rich text conversion and
vice versa, ensuring easier understanding and implementation for
developers.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/lib/markdown/convertRichTextToMarkdown.ts

@@ -0,0 +1,36 @@
+import { TElement } from '@udecode/plate-common'
+import serialize from './serializer/serialize'
+import { defaultNodeTypes } from './serializer/ast-types'
+
+export const convertRichTextToMarkdown = (
+  richText: TElement[],
+  options?: { flavour?: 'common' | 'whatsapp' }
+) => {
+  const test = richText
+    .reduce<string[]>((acc, node) => {
+      if (node.type === 'variable') {
+        return [
+          ...acc,
+          ...node.children.reduce<string[]>((acc, node) => {
+            const serializedElement = serialize(node, {
+              nodeTypes: defaultNodeTypes,
+              flavour: options?.flavour,
+            }) as string
+            if (!serializedElement || serializedElement === '<br>\n\n')
+              return [...acc, '\n']
+            return [...acc, serializedElement]
+          }, []),
+        ]
+      }
+      const serializedElement = serialize(node, {
+        nodeTypes: defaultNodeTypes,
+        flavour: options?.flavour,
+      })
+      if (!serializedElement || serializedElement === '<br>\n\n')
+        return [...acc, '\n']
+      return [...acc, serializedElement]
+    }, [])
+    .join('')
+
+  return test.endsWith('\n') ? test.slice(0, -1) : test
+}