chore: dedupe safe_not_equal in svelte/store

[Conduitry]

While looking at a bundled app that used stores, I noticed I had both safe_not_equal and safe_not_equal$1 - each having almost identical implementations. I don't think there's any reason for stores to need their own copy. This switches them over to using the same copy as is used elsewhere.

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[dummdidumm]

ha, by coincide I also noticed something related; this will give a merge conflict soon but you can easily resolve it once I merged the other dts-buddy PR