Update types & ember to 4.x

[mydea]

This PR updates all dev dependencies of types to their latest version, as far as possible.
It also updates the documentation for how to import/extend the TestContext type.

Closes #1470

[chriskrycho]

Thanks for this! I have been independently working on a similar branch, so this is great. I think it needs one more change: bumping the supported Node version everywhere it's specified (engines, CI, etc.) up to at least v12 – that will make the remaining CI jobs pass, I think.

[mydea]

I have updated node to 12 everywhere (and setup CI to use the pinned volta version, so this only needs to be set in a single place).

FYI I have also tried to update all other dev dependencies (basically yarn ember-cli-update etc.), but that lead to major issues with the tests. I think it has something to do with how ember-auto-import etc. works, and probably requires some more thought - e.g. the skeleton app for the build testing was not really outputting the given app/app.ts file at all, for whatever reason, but the generated dist/asets/skeleton-app.js file had a content like this:

'use strict';



;

;define('skeleton-app/config/environment', [], function() {
  var prefix = 'skeleton-app';
try {
  var metaName = prefix + '/config/environment';
  var rawConfig = document.querySelector('meta[name="' + metaName + '"]').getAttribute('content');
  var config = JSON.parse(decodeURIComponent(rawConfig));

  var exports = { 'default': config };

  Object.defineProperty(exports, '__esModule', { value: true });

  return exports;
}
catch(err) {
  throw new Error('Could not read config from meta tag with name "' + metaName + '".');
}

});

;
          if (!runningTests) {
            require("skeleton-app/app")["default"].create({});
          }
        
//# sourceMappingURL=skeleton-app.map

If there is anybody is interesting in looking at what I've done so far, you can see it here:
https://github.com/mydea/ember-cli-typescript/tree/fn/update-ember-cli