fix(js): quote property name as es5 compatible

[koba04]

Currently, Prettier quotes property names if the name isn't es6 compatible.
It means that Prettier might strip the quote for a property name that is es6 compatible but is not es5 compatible.
So this might break scripts on ES5 environments.

For example, the following example doesn't work on some ES5 environments.
You can try the script on Node v0.12.

Prettier 1.14.3
Playground link

--parser babylon

Input:

var obj = {
 "𐊧": 'ok'
};

console.log(obj);

Output:

var obj = {
  𐊧: "ok"
};

console.log(obj);

This PR is to fix this.

• 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)
• I’ve read the contributing guidelines.

[lydell]

I think this makes sense. Haven't we said before that by default we don't want to turn es5 into es6 – that's the reason we won't default trailing-comma to "all"?

I think using this kind of identifiers is not very common, so it should be a non-controversial change.

So 👍 from me.

[koba04]

Thank you!