support older JSC; fixes #2, closes #4, closes #1

[leonardpauli]

fixes #2, closes #4, closes #1

1. using babel-preset-env with targets safari 6 (with upgraded @babel/cli)
2. proper Function+arguments workaround

// func.apply(void 0, ...args) doesn't work with some
// special functions in older JSC versions (eg. OSX 10.11)
//  - use Function instead of eval
//    see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval
//  - beware of arguments gotchas
//    see https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments
const args = [structPtr, selector]; [].push.apply(args, arguments);
const argsStr = args.map((_, i)=> `args[${i}]`).join(',');
return Function(`"use strict";return function (fn, args) { return fn(${argsStr}) }`)()(func, args);

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I'm running El Capitan for "reasons", and many other have had this issue. This is a more robust and less hacky way than the previous attempts. My main issue which this solves (tested by npm link):

skpm/sketch-module-web-view#51 and
skpm/sketch-dev-tools#24

also: airbnb/react-sketchapp#106