Fix support for asynchronous plugins in rehypePlugins

[vishwamartur]

Related to #662

Refactor Markdown component to support asynchronous plugins.

• Markdown Component (core/src/components/TextArea/Markdown.tsx)

  • Import useState from React.
  • Add mdStr state to manage the processed markdown string.
  • Refactor useEffect to handle asynchronous processing of markdown using rehype().process.
  • Update the return statement to use mdStr state.

• Example Component (www/src/Example.tsx)

  • Import rehypePrettyCode.
  • Add rehypePrettyCode to previewOptions.rehypePlugins.

• Tests (test/editor.test.tsx)

  • Add a test to verify that MDEditor supports asynchronous plugins.
  • Use an example async plugin that adds a class to the first node.
  • Verify that the class is added to the preview node after processing.

[vercel[bot]]

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[JakeTrevor]

Any updates on this? I'd love to see it merged because I need to use async plugins.