Handle new versions of vite-plugin-svelte

[IanVS]

Fixes #381

There have been two changes in @sveltejs/vite-plugin-svelte that cause problems for us, which this PR addresses:

• https://github.com/sveltejs/vite-plugin-svelte/releases/tag/%40sveltejs%2Fvite-plugin-svelte%401.0.0-next.42 started to merge the svelte.config.js file by default, which may not be what storybook users want / expect, since their production config may be different from the storybook config (same reason we don't automatically merge vite.config.js). With the changes here, the configFile option in svelteOptions can still be set to true or to a specific file, if desired.

• https://github.com/sveltejs/vite-plugin-svelte/releases/tag/%40sveltejs%2Fvite-plugin-svelte%401.0.0-next.43 began returning an array of plugins rather than just a single one. This would normally be fine, except that we hack around with the plugin to create two separate ones, with different names, to avoid HMR in story files. So, this PR checks to see if the result is an array or not, and reacts accordingly. Note, I'm throwing away the inspector for story files. This might be a problem, but until we hear about it, I think this'll be fine. The inspector is still pretty experimental anyway, from what I can tell.

To test, you can revert the changes I've made in vite-config.ts, and you'll see that the svelte example breaks (I updated the svelte plugin in it). With the changes, it builds successfully.

This also enables the use of the new experimental inspection plugin.

To enable this feature, be sure you have @sveltejs/vite-plugin-svelte version 1.0.0-next.43 or higher, then add the following to your svelteOptions in .storybook/main.js:

module.exports = {
  // ... rest of config
  framework: '@storybook/svelte',
  svelteOptions: {
    preprocess: preprocess(),
    experimental: { inspector: true }, // <-- this line
  },
};

Then, when the storybook opens, focus a story iframe and press cmd+shift (mac) or ctrl+shift (windows) to activate.