fix: improve setup of default export and remove svelte/ssr

[benmccann]

I removed svelte/ssr, which will require vite-plugin-svelte 2.1.1. See sveltejs/vite-plugin-svelte#622 for the main PR there, which was released in 2.1.0 (minor bug fix came out in 2.1.1)

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[dummdidumm]

For future self:

svelte/ssr was added due to #6372 but only vite-plugin-svelte ever used it, because soon after we discovered that an export condition does the same (#6743). So it's safe to remove in Svelte 4 because it's highly unlikely anyone depends on this (at least not our Rollup/Webpack plugins)