Scenery build fails to load synchronously

[jonathanolson]

@samreid noted it may be related to phetsims/chipper#499.

This prevents documentation from being viewed, examples from being run, etc.

[jonathanolson]

It looks like the synchronous support in chipper was reverted in phetsims/chipper@001c895 (don't see a comment why).

It looks like asynchronous may be preferable/required for sims (maybe the progress indicator?), so I've added support for both ways of doing things.

Scenery has an added section in package.json that illustrates the changes:

  "phet": {
    "isJSOnly": true,
    "requiresLodash": true,
    "requiresJQuery": true,
    "assignGlobals": {
      "scenery": "main",
      "kite": "KITE/main",
      "dot": "DOT/main",
      "axon": "AXON/main",
      "phetCore": "PHET_CORE/main"
    },
    "finalizeJS": "scenery.Util.polyfillRequestAnimationFrame();"
  }

where options are documented in getBuildConfig (and only relevant when isJSOnly is true).

This also allows us to just "grunt" or "grunt build" for Scenery/Kite/Dot, since we can detect whether a repo is isJSOnly before creating the tasks, so I've done that (makes things easier).

This fixes the issues I was having (asynchronous loading, various scripts/files not working) that were blocking other issues.

@samreid, can you review the changes, and potentially suggest alternatives?

[samreid]

I skimmed the change sets and it all seems reasonable (particularly regarding synchronous/asychronous in the requirejs build), and I like how grunt builds js code for libs now.