Desktop: Resolves #8931: Improve support for plugins in the Rich Text Editor (implement webviewApi.postMesage)

[personalizedrefrigerator]

Summary

This pull request makes it possible for a plugin's content scripts to communicate with its background script, without using internal Joplin APIs (e.g. through parent.require). Specifically, it

1. Allows renderer plugins to access webviewApi when running in the rich text editor. This is the same API as is used in the markdown preview pane.
2. Updates tests/support/plugins/content_script/ to support the rich text editor.

Testing

1. Build the content_script example plugin and add it as a development plugin to Joplin.
   • It may be necessary to upgrade its version of TypeScript and Webpack to do this.
2. Create a note with the following content:

   Test
   ```justtesting
   foo
   bar
   baz
   ```

3. Switch to the Rich Text Editor.
4. Click on the generated link and check the console for a Got response in content script: justtesting+response message.
5. Make a change to the text just above the rendered block.
6. Switch back to the markdown editor
7. Verify that the original markup for the justtesting block is still present.