Add decorator support to defclass

[seanstrom]

Summary

• It could be nice to support decorator functions with the defclass syntax for fields, methods and classes.
• Ideally, there would be some way to interop with JS decorator functions with the defclass syntax.
• Note, it seems that decorators can be implemented in a variety of ways, and I'm not sure if there's a best way to standardise support, so more research may be required.

What are decorators?

• Here's the existing TC39 proposal: https://github.com/tc39/proposal-decorators
• Here's some documentation from TypeScript: https://www.typescriptlang.org/docs/handbook/decorators.html#class-decorators
• Here's an article describing decorators and providing some examples: https://www.sitepoint.com/javascript-decorators-what-they-are/#creatingcustomdecorators

How are Decorators used in JavaScript?

Decorators are functions that can modify the definition of class (and more) in JS, for example:

function decoratorName(opts) {
  return (target, key, descriptor) => {
    return descriptor
  }
}

class Example extends Foo {
  @decoratorName
  fieldName
}

Would be roughly de-sugared into:

let fieldDescriptor = Object.getOwnPropertyDescriptor(Example.prototype, "fieldName")

let propertyConfig = decoratorName(undefined)(Example.prototype, "fieldName", fieldDescriptor) 

Object.defineProperty(Example.prototype, "fieldName", propertyConfig)