Fix exports in index when using --filename-case option

[TimonVS]

Summary

Using the --filename-case option with kebab-case would break exports from the generated index file as it would use the filename as the export name. It doesn't break for PascalCase and camelCase, though in the case of camelCase, the export would also be camelCased even though it should always use PascalCase.

I'm not extremely happy with the solution, as I'd much rather abstract it away from defaultIndexTemplate, but that would require a breaking change which I don't think is worth it.

Another solution I was thinking about is using export * from './icon-name' (also requires a breaking change) but the generated icon component files are using export default and the component names include a prefix like Svg and/or Memo when using momoization etc. which the index file strips away.

Test plan

I've added snapshot tests :)

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[TimonVS]

I should've looked more closely at the open PRs 🤦 . Seems like #489 already fixes the issue.