chore: Use barrel files consistently in Lemon UI

[maxbelm]

This PR should make it so that all lemon-ui components can have their imports reference the @posthog/lemon-ui package.

The goal of this PR is to help with #16827 / #10471 -- this doesn't quite accomplish any of the listed work items, but should help move towards finishing

move lemon ui to their own package folder

mentioned in #10471

I'd be happy to help complete that work item, but I could use a bit of clarification about what that entails 😄

In my mind there's at least two more steps:

1. Moving the lib/lemon-ui folder/files under the @posthog/lemon-ui package directory
2. Update all the lib imports across the app to use the package import -- Given how many places this touches, feels like something to break down over multiple PRs?

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Note:

In updating the barrel file, I noticed some folders were exporting with a local barrel file, and others were not. It didn't seem like this was necessarily intentional, so I went ahead and made it more (not perfectly) uniform, making all the components have a barrel file of their own. There were also a couple of index.tsx files that were renamed to .ts files.

Please let me know if any of these changes aren't desired, or should be modified in some way!

[Twixes]

That's some really nice cleanup, thanks! You're correct with this approach and the follow-up steps – we just haven't been prioritizing this refactoring highly