Expose Core API's as classes using Typescript's "ES Private fields"

[rudolf]

From #52397 (comment)

With Typescript's ES Private field support being available in 3.8, we can expose the typings for Core's API's as real classes instead of the workarounds of using derived types or separate interfaces.

Phase 1: investigation

Using Typescript 3.8 Beta investigate this approach to ensure it works with our other tooling such as api-extractor and api-documenter.

Phase 2: rollout

• Bump Kibana's typescript to 3.8
• Replace all separate type or interface workarounds with a class

(marked as a blocker since we'd like to get this kind of API type churn done before 8.0.0)

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

@rudolf given the new structure that Core code follows (public type packages being fully dissociated from internal implementation packages), I think we may want to close this?

[rudolf]

Yes, I agree this is no longer relevant.