Precompress wasm files

[stephtr]

When enabling the precompress option, WebAssembly files currently aren't compressed.
In my case however, compression brings me down from 4 MB to 1.5 MB, which seems to be in line with what can be observed in the wild.

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[Rich-Harris]

Thank you. Rather than worrying about keeping these in sync, this logic probably belongs in Kit itself — will open a fresh issue for that

[stephtr]

Thank you too!