Ensure support for older versions of Typescript

[Tiagoperes]

We support Angular versions as early as 7.0. Angular 7.0 uses Typescript 3.1.1. In Beagle-Web-Core we use Typescript 3.7.5, which has many incompatibilities with TS 3.1.1.

To solve these incpompatibilies, we have to replicate some of the TS types and export them instead of using the native TS types. This way, they can be used by every TS version.

As of today (10/07/2020) our code is fully compatible. The problem is, it is not clear to us when we use a feature that is not supported by older versions of TS, or when TS compiles a code that is not compatible. Everything will work fine when we execute Beagle in projects using newer versions of TS, we'll see that the code is broken only when trying to run Beagle in a project using an old TS version.

We need a way to generate code that is guaranteed to be compatible or at least have an automatic way of verifying the compatibility.

We already tried to downgrade the TS version to 3.1.1, but it turned out to be a terrible idea, since many of our dependencies depend on newer versions of TS.

Related issues in Typescript:

• Automatic Backward/Forward .d.ts compatibility system microsoft/TypeScript#31907
• Provide a back-compat mechanism for bundled .d.ts files in package.json microsoft/TypeScript#22605