Preserve existing .gitignore file in out directory

[sinking-point]

Closes #984 .

Rather than check for an existing .gitignore before creating one, I opted to use create_new and check the error. This makes it atomic - it'll probably never matter, but hey.

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• [✅] You have the latest version of rustfmt installed

$ rustup component add rustfmt

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[andrewhamon]

This is much needed in a monorepo. We want to check some files (like d.ts) so that typescript can function as normal without needing to depend on a rust toolchain.