Async event handler for terrain providers

[ggetz]

We are deprecating readyPromises throughout the API and making a move towards an API which makes better use of async/await type patterns.

However, based on discussion in #11033, it would be convenient for most users to have a simpler API, while still preserving the .fromXYZ-style factory functions for those users who may prefer using async/await and simplifying code further down the stack.

In this approach, Terrain is a new object that handles the async error handling and exposes an error event and a ready event. One plus here is that this is more in line with event-driven UIs then readyPromise was. Viewer and CesiumWidget now take terrain options, which is then handled by Scene. The result is behavior much like what exists now in main, and prevents some common race conditions that could arise if use async/await.

• Passing config values to a Viewer or CesiumWidget constructors can begin rendering the globe immediately while waiting on a terrain provider promise in the background
• By default, errors are caught and logged to the console, but the user can subscribe to errorEvent to override this behavior
• The existing terrain immediately disappears when setting the new Terrain object, so that it's obvious that the update process has started
• Handles "competing" values, like when setting a terrain provider and then immediately setting another, or a terrainProvider directly
• Handles Viewer, CesiumWidget, or Scene being destroyed while a terrain update in in progress

One downside of this approach is that (without TypeScript) it's not possible to confirm that a promise resolves to the expected type during object creation. However, there are many other areas of the code where this is the case, and hopefully won't be a concern if/when we move to TypeScript.

TODO:

• Sweep other Sandcastle examples and update the API where applicable

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[jjhembd]

Can we move the helper functions down, so the constructor is at the top of the file?

[jjhembd]

Thanks @ggetz, this looks good! I just mentioned one nitpick about the ordering of a file.

[ggetz]

Thanks @jjhembd! I've responded to your feedback, and I've also gone ahead and swept through Sandcastle examples dealing with terrain. A few made sense to use await inside of a try/catch, but otherwise, especially where terrain is not the point of the example, I've changed to this new Terrain handler.

[jjhembd]

Looks good to me @ggetz! Merging now