Better TS support

[chriskrycho]

In line with the proposal at emberjs/rfcs#730:

• specify conformance (link to the as-yet un-finished RFC) – Introduce TS support policy with TS 4.4–4.6 #313
• define a support policy (should almost certainly be "simple majors") and specify it – Introduce TS support policy with TS 4.4–4.6 #313
• specify the supported TS versions and add type test coverage to CI – Introduce TS support policy with TS 4.4–4.6 #313, Add type tests and fix a types bug they exposed #314
• specify how this library defines "public API" (all exported type etc.)
• set strict: true and noPropertyAccessFromIndexSignature: true in tsconfig.json – Adopt SemVer spec strictness guidelines #315