Fix: object-shorthand providing invalid fixes for typescript

[bradzacher]

What is the purpose of this pull request? (put an "X" next to item)

[x] Bug fix

Related: typescript-eslint/typescript-eslint#124

The bug is that currently the fixer adds extra parentheses when fixing a TS arrow function with a return type:

/* eslint object-shorthand: [1, "always", { avoidExplicitReturnArrows: true }] */

const test = {
  key: (arg: any): any => {}
}

///// fixes to

const test = {
  key((arg: any): any) {}
}

This happens because the rule currently searches backwards from the => to find a ), to determine if it should wrap the arguments in parentheses. It obviously instead finds a return type annotation, hence the breakage.

What changes did you make? (Give an overview)

Adjusted the fix algorithm for arrow functions so that it searches from the start of the arrow function expression instead.

I added a TS-specific test for this - happy to remove it if you don't want the burden.

[platinumazure]

Raising one question.

I'm personally in favor of adding a test that simulates the Typescript case, but I'll defer to team consensus on that one.

[bradzacher]

I've refactored the code so it doesn't rely on a check for async to get the first token.

I've also added a test with typescript code to help prevent a regression.

[mysticatea]

LGTM, thank you!