Vite: Include user config optimizeDeps in final optimizeDeps

[agarun]

Issue:

What I did

currently: If a dev specifies optimizeDeps in their custom vite config, it will be ignored by @storybook/build-vite. I would consider this a bug, because a custom config should still be supported where possible

now: I spread optimizeDeps into the final object so it will update the config as expected

just for demonstration (my weird use case): here I create a custom vite config where optimizeDeps.esbuildOptions forces .js files to be read as .jsx

with this PR it will be included in the final config:

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?

If your answer is yes to any of these, please make sure to include it in your PR.

[yannbf]

Hey @agarun thanks a lot for your contributiont! LGTM