Repository: storybook
Issue: [Bug]: Nullish coalescing operator in Svelte component breaks docspage and argstable (sveltedoc-parser limitation)
Labels: bug,svelte

### Describe the bug

If a Svelte component contains a [nullish coalescing operator (`??`)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator) in its `<script>` block, the doc page for a story for the component has errors:

* Missing description (if set)
* Broken args table

(Many more details in the **Additional context** section.)

### To Reproduce

Reproduction: https://github.com/alecglassford/repro-storybook-svelte-nullish-coalescing

Preview: https://stackblitz.com/github/alecglassford/repro-storybook-svelte-nullish-coalescing

### System

```shell
The reproduction environment:

npmPackages:
  @storybook/addon-essentials: ^7.0.0-rc.4 => 7.0.0-rc.5 
  @storybook/addon-interactions: ^7.0.0-rc.4 => 7.0.0-rc.5 
  @storybook/addon-links: ^7.0.0-rc.4 => 7.0.0-rc.5 
  @storybook/blocks: ^7.0.0-rc.4 => 7.0.0-rc.5 
  @storybook/svelte: ^7.0.0-rc.4 => 7.0.0-rc.5 
  @storybook/svelte-vite: ^7.0.0-rc.4 => 7.0.0-rc.5 
  @storybook/testing-library: ^0.0.14-next.1 => 0.0.14-next.1 

But also, I first encountered the bug in the following environment:

npmPackages:
  @storybook/addon-actions: ^6.5.13 => 6.5.13 
  @storybook/addon-essentials: ^6.5.13 => 6.5.13 
  @storybook/addon-interactions: ^6.5.13 => 6.5.13 
  @storybook/addon-links: ^6.5.13 => 6.5.13 
  @storybook/addon-svelte-csf: ^2.0.10 => 2.0.10 
  @storybook/builder-webpack5: ^6.5.13 => 6.5.13 
  @storybook/manager-webpack5: ^6.5.13 => 6.5.13 
  @storybook/preset-scss: ^1.0.3 => 1.0.3 
  @storybook/svelte: ^6.5.13 => 6.5.13 
  @storybook/testing-library: ^0.0.13 => 0.0.13
```


### Additional context

This occurs because [sveltedoc-parser](https://github.com/alexprey/sveltedoc-parser/), which Storybook uses to parse documentation details from Svelte components, only supports parsing ES9 (aka ECMAScript 2018) syntax in Svelte compoents  (https://github.com/alexprey/sveltedoc-parser/blob/7b7d0ff777731ce8ed35ddead97eaa85afe9e82f/lib/options.js#L234) and the dependency throws an error if you use it to parse JavaScript with language features not supported in ES 9 (many of which, like the nullish coalescing operator, are supported in Svelte and many modern browsers). This likely means that [numerous other post-ES9 language features](https://caniuse.com/sr_es10,sr_es11,sr_es12,sr_es13) also break the docs page; the nullish coalescing operator is just the one I encountered.

Changing the single line of code linked in the preceding paragraph to support `ecmaScript: 11` fixes the bug (see reproduction); unfortunately, sveltedoc-parser does not make this option configurable and the project does not seem to be actively maintained.

A few possible resolutions come to mind:

* Someone files an issue upstream with `sveltedoc-parser`, but, as mentioned, the project does not seem to be maintained (no activity for over a year), so this might not work.
* Storybook forks the sveltedoc-parser repository and maintains a new version of it. This is a tall order, and I don't know if the Storybook team has the capacity for it (/I wouldn't necessarily expect you to). That said, [Storybook seems to be the only project actively using the sveltedoc-parser npm package](https://www.npmjs.com/package/sveltedoc-parser?activeTab=dependents), so y'all are as well positioned for it as anyone. 😄 
* Storybook finds (or makes) a different tool to parse documentation details from Svelte components.