Svelte: Improve default webpack config

[j3rem1e]

Issue: #14221

What I did

Added 'svelte' in the webpack mainFields, in the first position.

How to test

• Is this testable with Jest or Chromatic screenshots? No
• Does this need a new example in the kitchen sink apps? No
• Does this need an update to the documentation? No

[shilman]

LGTM .. i guess? :)

[j3rem1e]

I don't know why, but mainFields was set when running storybook but not with build-storybook.

Moreover, I couldn't use your suggestion, because when mainFields is null, it uses default values, but not when it's set. I can't just add 'svelte' without adding the default values (I had compilation errors without).