ES6ify Two.js Codebase

[jonobr1]

🎯 Affects issues:

1. Use ES6 features in the codebase? #509
2. [Bug] Cannot import two.js in typescript project #531
3. [Enhancement] Add return type for vector functions #542

📋 This is a more-or-less entire rewrite of Two.js to use EcmaScript 6 features. In particular:

• const / let instead of var for tighter scoping
• class construction instead of prototypical modifications
• Expanded classification of classes. For instance:
  • Two.Events is now a class
  • Two.Element is a new base class of Two.Shape, Two.Gradient, and anything else that can be queried in the scenegraph
• Where possible functions are named instead of anonymous
• Removes all MakeObservable methods in favor of Object.defineProperty invocations on constructor
• Module imports are through typical exports and except for the root Two.js class, not with default. So you'll need to import specific modules like so:

import { Vector } from 'two.js/src/vector.js';
var v = new Vector();

🏁 These changes allow for improved:

• TypeScript Declarations (fully expanded and invoked through TypeScript's types compiler)
• Improved documentation
• Code legibility and OOP style
• Performance debugging
  • Easier to identify culprit functions in Chrome et al. performance debug console

⚠️ These changes break:

• Loose interoperability between Two.Vector and Two.Anchor. For any curve, it's required you use anchors instead of vectors now.

🗒️ All tests and first party examples are passing

[lgtm-com]

This pull request introduces 9 alerts when merging 891d132 into fc7f667 - view on LGTM.com

new alerts:

• 3 for Superfluous trailing arguments
• 1 for Useless assignment to local variable
• 1 for Useless conditional
• 1 for Variable not declared before use
• 1 for Wrong use of 'this' for static method
• 1 for Unused variable, import, function or class
• 1 for Access to let-bound variable in temporal dead zone

[lgtm-com]

This pull request introduces 1 alert when merging 6a16043 into fc7f667 - view on LGTM.com

new alerts:

• 1 for Useless conditional