Improve/refactor .cancel()

[ehmicky]

This separates cancel() into its own function.

Also this makes cancel() return a boolean like kill() does. The fact that kill() returns a boolean is not documented and is not part of @types/node so I think we should do the same, and not document it nor type it. However it would not hurt to still forward kill() return value.

[sindresorhus]

The fact that kill() returns a boolean is not documented

You should open a Node.js issue about either documenting it or removing that return value.

However it would not hurt to still forward kill() return value.

I disagree. It breaks the least amount of surprise rule. We should not do undocumented stuff. I think we should not return a boolean. If anyone needs it, they will request it and we can consider adding it officially.

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So I don't think cancel should return anything either, at least for now.

[ehmicky]

Fixed.