create-svelte: Use target es2020 as default in tsconfig.json

[ambarvm]

fixes #2457

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Latest commit: 61c0204

The changes in this PR will be included in the next version bump.

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create-svelte	Patch

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[dummdidumm]

We have not done this in the past because some we support Node 12.x, and that doesn't support es2020 features, so it would be confusing for new users if they run into a situations where their deploy target does not support the newer features which are not transpiled if the target is es2020. More info in #772 .
I'm therefore slightly in favor of not merging this.

[Conduitry]

Yup, agreed, I don't think we want to do this.