Desktop: Fixes #9752: CodeMirror 6 plugin API: Allow importing @codemirror/lang-markdown and @lezer/highlight

[personalizedrefrigerator]

Summary

Allows plugins to import @codemirror/lang-markdown and @lezer/highlight without also bundling additional copies of the libraries.

Testing

1. Create a plugin that uses the updated webpack.config.js (I copied webpack.config.js into an existing plugin).
2. Require both @codemirror/lang-markdown and @lezer/highlight
3. Inspect the generated bundle to verify that neither library has been included
4. Run the plugin and verify that both libraries were imported

[personalizedrefrigerator]

While @codemirror/highlight exists, it's deprecated and split into @lezer/highlight and @codemirror/language and thus should not be used.