Test code is from #6657 (by Cryrivers), replace @typescript-eslint/typescript-estree v2.4.0 with my fork version that works for node 4

• Optional Chaining (Fixes [TypeScript 3.7] add support for optional chaining #6595)
• Null Coalescing (Fixes [TypeScript 3.7] add support for nullish coalescing operator #6609)
• asserts keyword (Fixes [TypeScript 3.7] add support for asserts keyword #6574)
• optional computed class property (Fixes [Typescript] Prettier removes question mark from optional computed class property #6569) , was fixed in Typescript: Keep question mark for optional computed class property #6601 with same solution

I suggest we use my fork until prettier starting v2.0.0(will drop node 6 support #3919), since many people want Typescript 3.7 support. we can switch back anytime

Yesterday, I saw someone from typescript-eslint team says they may try to bring node 6 support back, hope it's true.

My build script can be found on https://github.com/fisker/typescript-estree-for-node-6 , repo is a little mess now

• script: https://github.com/fisker/typescript-estree-for-node-6/blob/9d59ec8e0a8204819d0822b6fac136fda6d3f339/package.json#L14-L16
• rollup config https://github.com/fisker/typescript-estree-for-node-6/blob/master/rollup.config.js
• babel config https://github.com/fisker/typescript-estree-for-node-6/blob/master/babel.config.js
• code replace https://github.com/fisker/typescript-estree-for-node-6/blob/master/scripts/fix-code.js
• package.json build https://github.com/fisker/typescript-estree-for-node-6/blob/master/scripts/sync-version.js

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• (If the change is user-facing) I’ve added my changes to the CHANGELOG.unreleased.md file following the template.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨