fix: streamline SWC options handling in webpackFinal

[valentinpalkovic]

Closes #10
Closes #7

Storybook documentation suggests you can customize the swc config, by a function that accepts the default config:

swc: (config: Options, options): Options => {
    return {
      ...config,
      // Apply your custom SWC configuration
    };
  },

config is an empty object, though. In this PR, deep merging the user's settings with the SWC default options no longer happens. Instead, the default SWC loader settings are passed as options into the swc preset function, and the user is responsible for returning the whole SWC configuration from the preset hook. This can lead to a breaking change!

📦 Published PR as canary version: 2.0.0--canary.13.b38e7cf.0

✨ Test out this PR locally via:

npm install @storybook/[email protected]
# or 
yarn add @storybook/[email protected]