Fixes#20694
Implements super basic new platform `core` system, which includes two services: `core.injectedMetadata` and `core.legacyPlatform`. The `core` currently has two responsibilities:
1. read the metadata from the DOM and initialize the `ui/metadata` module with legacy metadata, proving out how we plan to expose data from the new platform through the existing APIs/modules to the legacy platform.
2. bootstrap the legacy platform by loading either `ui/chrome` or `ui/test_harness`
Because `core` mutates the `ui/metadata` module before bootstrapping the legacy platform all existing consumers of `ui/metadata` won't be impacted by the fact that metadata loading was moved into the new platform. We plan to do this for many other services that will need to exist in both the legacy and new platforms, like `ui/chrome` (see #20696).
This upgrades TypeScript to version 2.9.2. My main motivation is the support for generic type arguments in JSX elements and tagged templates (e.g. for `styled-components`).
Problems arising from breaking changes in the new TypeScript version have been mitigated by:
* setting the `keyofStringsOnly` option until impacted code has been
future-proofed
* Restricting some joi-related generics
See the [release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html) for details.
Some follow-up tasks should be:
* Update EUI to support the new `keyof` behaviour
* Update new platform TypeScript code to support new `keyof` behaviour
* Remove `keyofStringsOnly` setting
* Typescriptify ui/embeddable folder
* Allow ts modules from ui/public folders to be imported
* Address review comments from Tim
* Address code review from Oleg
* remove lodash usage
* remove mappng to non public folder
