fix: neon new not exiting on windows

[amilajack]

Fixes #406
Closes #501

This bug fixes a bug where running neon new on windows will hang without ever exiting despite creating a new repo and all.

Changes

• Migrates to use native fs module
• Bumps TS to latest version
• Adds make-promises-safe to CLI entrypoint to ensure promise do not silently fail
• Build targets ES6
• Removes node 8 from CI, adds node 14
• Removes ts-unknown and es6-polyfill dependencies
• Change methods to sync

Testing this PR

Make sure you're on a windows machine to test this PR

git clone https://github.com/neon-bindings/neon test-repo
cd test-repo
npm i
npm run transpile
node bin/cli.js new foo

This fix is based on @usagi 's PR (#501). Thanks!

[goto-bus-stop]

Other than the question about unlink, this looks good to me! Works as expected on my Win10 machine. Love the addition of make-promises-safe too :)

[dherman]

Looks great! I just have two tiny suggestions about the use of existsSync, but they're up to you.