clang-format: [JS] options for arrow functions.

Summary:
clang-format currently always wraps the body of non-empty arrow
functions:

    const x = () => {
      z();
    };

This change implements support for the `AllowShortLambdasOnASingleLine`
style options, controlling the indent style for arrow function bodies
that have one or fewer statements. SLS_All puts all on a single line,
SLS_Inline only arrow functions used in an inline position.

    const x = () => { z(); };

Multi-statement arrow functions continue to be wrapped. Function
expressions (`a = function() {}`) and function/method declarations are
unaffected as well.

Reviewers: krasimir

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73335