Simplify mdxPlugin used in vite builder

[IanVS]

Issue:

We are currently adding @vitejs/plugin-react to every non-react vite storybook project, in order to process the code that is returned from @storybook/mdx2-csf.

What I did

This change pulls in storybookjs/mdx2-csf#20, which swaps out JSX syntax for a call to the _jsx() transform instead, which is what the rest of the code returned from @mdxjs/mdx uses.

That allows us to remove the use of the react vite plugin in our custom mdx vite plugin.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

I'm not super-confident in this change, and I'm hoping CI will help give a bit of confidence. But it will need a close look as well. I tested in a react sandbox, but the mdx there is not very complex.

[ndelangen]

@IanVS is this PR still relevant? I know there's a bunch of other PRs going on regarding MDX, @shilman

[IanVS]

I think this is cleanup that we'll want to do at some point, yes, but I'm unclear on when it should happen.

[ndelangen]

@IanVS I think this statement:

We are currently adding @vitejs/plugin-react to every non-react vite storybook project

... isn't true anymore, is it?

What do you propose we do to this PR?

[IanVS]

I think this is no longer needed.