RFC: Official TypeScript support

TypeScript has become a key part of the front-end development ecosystem
over the past several years, and powers many of the best developer
experiences in the front-end ecosystem. Ember was a relatively early
TypeScript adopter for its internals, and there is widespread usage in
the ecosystem with community support, but to date Ember has not provided
“out of the box” or official support for authoring apps or addons in
TypeScript.

This RFC declares our intent to make TypeScript a first-class citizen of
the Ember ecosystem, as a peer to JavaScript, in a way which makes the
developer experience better for *all* Ember developers. It outlines the
key constraints and goals for the effort, details a roadmap for
accomplishing those goals, and provides the following definition of
official support (from the Detailed Design: Defining Official Support
section in the RFC text):

> Ember officially supporting TypeScript means: _**All libraries which
> are installed as part of the default blueprint must ship accurate and
> up-to-date type definitions for the current edition. These types will
> uphold a Semantic Versioning commitment which includes a definition of
> SemVer for TypeScript types as well as a specification of supported
> compiler versions and settings, so that TypeScript will receive the
> same stability commitments as the rest of Ember.**_