feat: expose symbol registry and factory

[cbt1]

This enables component developers to use existing symbols and register their own symbols.

Register a new symbol

// In this example we'll do a dot symbol. We'll represent it as a circle with a static radius.
// x and y is the centroid of a square, where the length of the sides are represented by the size parameter. 
const symbolFunction = ({ x, y, size, ...additionalOptions }) => {
 return {
  type: 'circle',
  cx: x,
  cy: y,
  r: 2
 };
};

picasso.symbol.add('<name-of-symbol>', symbolFunction);

Using the symbol factory in a custom component

const myComponent = {
 require: ['symbol'], // Require the symbol factory, after which it is exposed on the this context.
 render() {
  const mySymbol = this.symbol({ type: '<name-of-symbol>', x: 10, y: 15, size: 20 });
  return [mySymbol];
 }
};

Implements: #197

As stated in #197, there is more work needed to fully implement support for this, for example adding documentation.