No "engines" key provided

[npdev453]

Project use optional chaining and recursive flag in mkdirSync that supported only in Node 10+ (or 12+)

It thinks it will be more clearly that engines property will be provided in packages.json

https://docs.npmjs.com/cli/v7/configuring-npm/package-json#engines

At node 8 I got:

./copy-files.hook.js:14
    api.tsconfig.include = api.tsconfig.include?.filter(file => file.endsWith('ts') || file.endsWith('*'));
                                                ^
    SyntaxError: Unexpected token .

Also, I think that a project like this should support the same engines that parent project support, for typescript is node 4.3+
to extend the round of potential users much as possible.

https://github.com/microsoft/TypeScript/blob/main/package.json#L28