Automatically detect use of p5.dom and p5.sound

[toolness]

I thought it might be useful to statically analyze the user's sketch and determine whether they're using p5.dom and/or p5.sound, and include those libraries in the preview frame if so. This has the benefit of resolving #58 without needing any extra setup or configuration on the part of the author.

escope could make the static analysis task pretty easy. (It could also make implicit-sketch.ts a lot simpler, too.)

Concerns:

• This will require us to have a representation of the global symbols of p5.dom/p5.sound in the widget source code, and they could go out of date.
• I'm not sure how much extra weight escope will add to the bundled code.

Below is an example snippet I wrote while playing around with escope.

Example escope snippet

"use strict";

let esprima = require('esprima');
let escope = require('escope');

let ast = esprima.parse(`
  var a = 1;

  function boop() {
    a += b;
  }

  boop
`);

let scopeManager = escope.analyze(ast);

function isInScope(ref, scope) {
  if (scope.set.has(ref.identifier.name)) {
    return true;
  }
  if (!scope.upper) {
    return false;
  }
  return isInScope(ref, scope.upper);
}

scopeManager.scopes.forEach(scope => {
  scope.references.forEach(ref => {
    console.log(ref.identifier.name, isInScope(ref, scope));
  });
});