use async writeFile (instead of writeFileSync) to speed up sapper export's save fn

[swyxio]

While investigating this issue: #893 i found that this PR dramatically sped up writing of data files, reducing the likelihood that child process termination would cut short the writing of data.

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Tests

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[khrome83]

@sw-yx - i think #869 solves this. Even with writeFile being used you can still get out of memory and block i/o.

[swyxio]

thank you. i see it now. i'll close this PR if that one is accepted, else this one has a smaller footprint. i think @nealalpert can use util.promisify there, its a part of the platform!

[antony]

@sw-yx going to close this and focus on getting #869 over the line :)