React: Lazy import react-docgen-typescript-plugin

[tmeasday]

This can take up to a second on my machine (it has a dependency on typescript), and is totally wasted effort if we are not using the plugin.

What I did

Just use an await import to import when needed.

We might look at what other "optional" presets are doing similar things.

How to test

Let's check docgen still works OK on some react projects. I think that should be covered by the CI tests.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[shilman]

Looks great and @ndelangen 's fix seems to work! 🚀